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CONVERSATIONS 

OK 
IN  WHICH 

THE  ELEMENTS  OF  THAT  SCIENCE 

ARE  FAMILIARLY  EXPLAINED, 

AND 

ADAPTED  TO  THE  COMPREHENSION  OF 
YOUNG  PUPILS. 

ZZiI-VSTRATSB   WZTH   FZ.ATES. 

BT  THE  AUTHOR  OF  CONVERSATIONS  ON  CHEMISTRY,  AND  . 
CONVERSATIONS  ON  POLITICAL  ECONOMY. 

IMPROVED  BY 

APPROPRIATE  QUESTIONS, 

FOR    THE    EXAMINATION    OF    SCHOLARS  ; 

ALSO  BY 

AND 

A  DICTIONARY  OF  PHILOSOPHICAL  TERMS. 


BY  REV.  J.  L.  BLAKE,  A.  M. 

Rector  of  St  Matthew's  Church,  and  Principal  of  a  Literary  Seminary, 
Boston,  Mass. 


EIGHTH  AMERICAN  EDITION. 

fSonton : 

PRINTED  AND  PUBLISHED  BY  LINCOLN  &.  EDMANDS, 

No.  59,  Washington-street,  (53,  Comhill.) 

STEREOTYPED  BY  T.  H.  CARTER  &  CQ.  BOSTON. 

1826. 


DISTRICT  OF  MASSACHUSETTS,  fofot*.. 

District  ClerVs  Ofiet, 
BE  IT  REMEMBERED,  that  on  the  fourth  day  of  December,  A.  D.  1824,  in 
the  forty-ninth  year  of  the  independence  of  the  United  States  of  Araericaj  JOHN 
LAURIS  BLAKE,  of  the  said  district,  has  deposited  in  this  office  the  title  of  a 
book,  the  right  whereof  he  claims  as  author,  in  the  words  following,  to  wit : 

"Conversations  on  Natural  Philosophy,  in  which  the  elements  of  that  science 
•re  familiarly  explained,  and  adapted  to  the  comprehension  of  young  pupils. 
Illustrated  with  plates.  By  the  author  of  Conversations  on  Chemistry,  and  Con- 
Tersations  on  Political  Economy.  Improved  by  appropriate  Questions,  for  the 
examination  of  Scholars ;  also  by  Illustrative  Notes,  and  a  Dictionary  of  Philoso- 
phical Terms.  By  J.  L.  BLAKE,  A.  M.  Rector  of  St.  Matthew's  Church,  and 
J*rincipal  of  a  Literary  Seminary,  Boston,  Mass." 

In  Conformity  to  the  Act  of  the  Congress  of  the  United  States,  entitled,  "  An 
Act  for  the  encouragement  of  learning,  by  securing  the  copies  'of  maps,  charts^ 
and  books,  to  the  authors  and  proprietors  of  such  copies  during  the  times  therein 
mentioned ;"  and  also  to  an  Act,  entitled  "  An  Act  supplementary  to  an  Act, 
entitled  An  Act  for  the  encouragement  of  learning,  by  securing  the  copies  of 
maps,  charts,  and  books,  to  the  authors  and  proprietors  of  such  copies  during  tha 
times  therein  mentioned;  and  extending  the  benefits  thereof  to  the  arts  of  de- 
sifoiog)  engraving,  and  etehing  historical,  and  other  prints." 

TNO   W   nAVT<5    ^  Clerk  of  the  District 
JNO.  W.  DAVIS,   J     of  Massachusetts. 


PREFACE. 


The  following  work  does  not  probably  contain  so  much  of  th« 
science  of  Natural  Philosophy  as  might  be  crowded  into  a  volume  of 
equal  size,  on  some  different  plan.  The  author  seems  to  have  been 
influenced  chiefly  by  other  considerations  ;  and,  in  the  opinion  of 
the  editor,  with  the  most  happy  success.  Mr^.  Bryan  did  not 
profess  to  prepare  a  work  suited  to  the  highest  stages  of  education. 
Her  aim  was  to  accommodate  an  important  science  to  the  literary 
taste  and  intellectual  apprehensions  of  persons,  within  whose  reach 
Natural  Philosophy  had  not  previously  been  placed — to  accommo- 
date to  the  use  of  schools  generally  a  science,  wiiich  had  hitherto 
been  considered  too  abstruse  and  uninteresting  for  any,  whose 
minds  had  not  been  disciplined  and  invigorated  by  long  and  regu- 
lar habits  of  study.  Instead  of  exhausting  the  intellectual  ener- 
gies of  youth  in  committing  to  memory  definitions  and  roathemati- 
eal  demonstrations,  which  would  nat  be  understood,  she  proposed 
to  illustrate  the  great  principles  of  Natural  Philosophy  by  compari- 
sons of  the  most  familiar  kind  ;  and,  it  is  believed,  Mrs.  Bryan  has 
done  more,  in  this  way,  towards  giving  youth  a  taste  for  the  study 
0f  philosophy  than  all  others  who  have  published  treatises  on  the  sub- 
ject. In  her  preface  she  remarks:*—"  It  is  with  increased  diflSdence 
that  the  author  offers  this  little  work  to  the  publick.  The  encou- 
raging reception  which  the  Conversations  on  Chemistry  and  Politi- 
eal  Economy  have  met  with  has  induced  her  to  venture  on  publish- 
ing a  short  course  on  Natural  Philosophy.  They  are  intended,  in  a 
course  of  elementary  science,  to  precede  the  Conversations  on 
Chemistry,  and  were  actually  written  previous  to  either  of  her  othef 
publications.** 

The  Conversations  on  Natural  Philosophy  were  introduced  into 
the  editor's  Seminary  about  three  years  since,  then  at  Concord, 
N.  H.;  but  it  was  soon  found  that  his  pupils  were  often  embar- 
rassed in  not  knowing  to  what  particular  parts  they  were  chiefly 
to  direct  the  attention,  committing  to  memory  what  was  not  neces- 
sary and  omitting  what  was,  thereby  causing  great  loss  of  time  as 
well  as  of  improvement.  This  induced  him  to  prepare,  as  they 
were  needed,  day  after  day,  Questions  for  their  examination. 
When  questions  were  thus  prepared  upon  the  whole  work,  it  wa» 


judged  expedient  to  hare  them  published  in  a  pamphlet,  which 
was  accordingly  done  ;  but  being  prepared  in  haste  and  without 
thought  of  their  being  published,  they  were  of  course  imperfect  j 
nor  was  there  opportunity  to  revise  them,  when  afterwards  printed 
with  notes  in  connexion  with  the  work  itself!  But  as  successive 
f  ditions  were  required,  and  as  the  demand  is  still  increasing,  he  has 
been  induced  to  revise  and  write  them  anew,  placing  them  at  the 
bottom  of  the  several  pages  to  which  they  relate ;  and,  also  to  in- 
•rease  the  number  of  Notes,  and  to  add  to  the  volume  a  Dictionary 
of  Philosophical  Terms. 

As  the  work  is  now  presented  to  the  publick,  the  Editor  has 
full  confidence  in  recommending  it  to  Instructors,  well  persuaded 
it  will  lessen  their  own  labour  and  facilitate  the  improvement  of 
their  pupils.  It  is  perfectly  obvious,  that,  instead  of  embodying, 
the  questions  at  the  close  of  the  book,  as  in  former  impressions 
great  convenience  will  be  found,  both  by  instructors  and  scholars,  in 
having  them  printed  on  the  pages  from  which  they  are  to  be  an- 
swered ;  nor  is  the  1?  Jour  of  finding  the  answers  to  be  given  so  les- 
sened, as  to  enable  scholars  to  select  those  answers  without  read- 
ing and  studying  the  whole  book. 

.  It  has  been  thought  best  to  place  the  Plates  at  the  end  of  the 
volume.  If  interspersed  throughout  the  work,  as  in  former  edi- 
tions, it  is  evident  that  no  more  than  one  page  could  face  eack 
Plate,  while  a  very  considerable  number  of  pages  would  have  re- 
ference to  it,  BO  that  the  object  contemplated  could  only  in  a  smali 
degree  be  accomplished.  Besides,  it  is  judged  advisable  by  the 
editor,  that  the  plates  should  not  face  the  explanations  in  the 
Text  if  practicable.  Many  of  the  Questions  are  to  be  answered 
from  the  Plates  ;  but  if  the  several  Plates  were  placed  opposite  the 
different  portions  of  the  work  to  which  they  relate,  the  answers 
might  be  read  from  the  explanations  there  given  instead  of  being^^ 
recited  from  the  figures  as  intended. 

J.  L.  BLA&E. 
Boston,  December  f  1824. 


CONTENTS- 


CONVERSATION  L 

On  General  Properties  of  Bodies, 

Introduction  ;  General  Properties  of  Bodies  ;  Impenetrability; 
Extension  ;  Figure ;  Divisibility  ;  Inertia  ;  Attraction ;  Attrac- 
tion of  Cohesion  ;  Density  ;  Rarity  ;  Heat ;  Attraction  of  Gra- 
vitation. Page  9. 

CONVERSATION  II. 

On  the  Attraction  of  Gravity, 

Attraction  of  Gravitation  continued  ;  Of  Weight ;  Of  the  fall  of 
Bodies;  Of  the  resistance  of  the  Air  ;  Of  the  Ascent  of  Light 
Bodies.  Page  24. 

CONVERSATION  III. 

On  the  haws  of  Motion, 

Of  Motion  ;  Of  the  Inertia  of  Bodies  ;  Of  Force  to  produce  Mo- 
tion ;  Direction  of  Motion ;  Velocity,  absolute  and  relative ; 
Uniform  Motion ;  Retarded  Motion  ;  Accelerated  Motion  ;  Ve- 
locity of  Falling  Bodies  ;  Momentum  ;  Action  and  Re-action 
equal;  Elasticity  of  Bodies  ;  Porosity  of  Bodies ;  Reflected  Mo- 
tion ;  Angles  of  Incidence  and  Reflection.  Page  36. 

CONVERSATION  IV. 

On  Compound  Motion. 

Compound  Motion  the  result  of  two  opposite  forces;  Of  Circular 
Motion,  the  result  of  two  forces,  one  of  which  confines  the  body 
to  a  fixed  point ;  Centre  of  Motion,  the  point  at  rest  while 
the  other  parts  of  the  body  move  round  it ;  Centre  of  Magnitude 
the  middle  of  a  body  ;  Centripetal  Force,  that  which  confines 
A  body  to  a  fixed  central  point ;  Centrifugal  Force,that  v/hich  im- 
pels a  body  to  fly  from  the  centre.;  Fall  of  Bodies  in  a  Parabola ; 
Centre  of  Gravity,  the  Centre  of  Weight,  or  point  about  which 
the  parts  balance  each  other.  Page  51. 

I  * 


yi  CONTENTS. 

CONVERSATION  V. 

On  the  Mechanical  Powers, 

Of  the  Power  of  Machines  ;  Of  the  Lever  in  general ;  Of  the  Le- 
ver of  the  first  kind,  having  the  Fulcrum  betv/een  the  Power  and 
the  weight ;  Of  the  Lever  of  the  second  kind,  having  the 
weight  between  the  power  and  the  Fulcrum  ;  Of  the  Lever 
of  the  third  kind,  having  the  power  between  the  Fulcrum  and 
the  Weight ;  Of  the  Pulley  ;  Of  the  Wheel  and  Axle  ;  Of  the 
inclined  Plane  ;  Of  the  Wedge  ;  Of  the  Screw,     Pages  60,  68. 

CONVERSATION  VI. 

ASTRONOMY. 

Causes  of  the  Earth's  Annual  Motion. 

Of  the  Planets,  and  their  motion  ;  Of  the  Diurnal  motion  of  the 
Earth  and  Planets.  Page  7&. 

CONVERSATION  VII. 

On  the  Planets. 

Of  the  Satellites  or  Moons  ;  Gravity  diminishes  as  the  square  of 
the  Distance;  Of  the  Solar  System;  Of  Comets;  Constellations^ 
sjgne  of  the  Zodiack  ;  Of  Copernicus,  Newton,  &c.     Page  9&, 

CONVERSATION  VIII. 

On  the  Earth. 

Of  the  Terrestrial  Globe ;  Of  the  Figure  of  the  Earth ;  Of  the 
pendulum  ;  Of  the  Variation  of  the  Seasons  j  and  of  the  Length 
of  Days  and  Nights ;  Of  the  causes  of  the  Heat  of  Summer  j 
Of  Soikr^  Siderial,  and  Equal  or  Mean  Time.  Page  102. 

CONVERSATION  IX. 

On  the  Moon. 

Of  the  Moon's  Motion ;  Phases  of  the  Moon ;  Eclipses  of  the 
Moon  ;  Eclipses  of  Jupiter's  Moons ;  Of  the  Latitude  and  Longi- 
tude ;  Of  the  transits  of  the  inferior  Planets  ;  Of  the  Tides. 

Page  124. 


CONTENTS.  VU 

CONVERSATION  X. 

HYDROSTATICKS. 
On  the  Mechanical  Properties  of  Fluids. 

Definition  of  a  Fluid  ;  Distinction  between  Fluids  and  Liquids ; 
Of  Non-Elastic  Fluids,  scarcely  susceptible  of  Compression  ; 
Of  the  Cohesion  of  Fluids  ;  Of  their  Gravitation  ;  Of  their  Equi- 
librium :  Of  their  Pressure ;  Of  Specifick  Gravity  ;  Of  the 
Specifick  Gravity  of  Bodies  heavier  than  Water  ;  Of  those  of 
the  same  weight  as  Water  ;  Of  those  lighter  than  Water  ;  Of 
the  Specifick  Gravity  of  Fluids.  Page  137. 

CONVERSATION  XL 

Of  Springs,  Fountains^  8^c. 

Of  the  Ascent  of  Vapour  and  the  Formation  of  Clouds  ;  Of  the 
Formation  and  Fall  of  Rain,  &c. ;  Of  the  Formation  of  Springs  ; 
Of  Rivers  and  Lakes ;  Of  Fountains.  Page  159. 

CONVERSATION  XIL 

PNEUMATICKS. 

On  the  Mechanical  Properties  of  Air. 

Of  the  Spring  or  Elasticity  of  the  Air  ;  Of  the  Weight  of  the  Air ; 
Experiments  with  the  Air  Pump  ;  Of  the  Barometer  ;  Mode  of 
Weighing  Air ;  Specific  Gravity  of  Air  ;  Of  Pumps  ;  Descrip- 
tion of  the  Sucking  Pump  ;  Description  of  the  Forcing  Pump. 

Page  158. 

CONVERSATION  XIIL 

On  Wind  and  Sound. 

Of  Wind  in  General;  Of  the  Trade  Wind;  Of  the  Periodical 
Trade  Winds  ;  Of  the  Aerial  Tides ;  Of  Sound  in  General ; 
Of  Sonorous  Bodies  ;  Of  Musical  Sounds ;  Of  Concord  or  Har- 
mony, and  Melody.  Page  170. 

CONVERSATION  XIV. 

On  Optics. 

Of  Luminous,  Transparent,  and  Opaque  Bodies ;  Of  the  Radiation 
of  Light ;  Of  Shadows  ;  Of  the  Reflection  of  Light ;  Opaque 
Bodies  seen  only  by  Reflected  Light ;  Vision  Explained  ;  Ca- 
mera Obacura  ;  Image  of  Objects  on  the  Retina.        Page  183 


▼Ui  CONTENTS. 

CONVERSATION  XV. 

On  the  Angle  of  Vision,  and  Reflection  of  Mirrors. 

Angle  of  Vision  ;  Reflection  of  Plain  Mirrors  ;  Reflection  of  Con- 
vex Mirrors  ;  Reflection  of  Concave  Mirrors.  Page  197. 

CONVERSATION  XVI. 

On  Refraction  and  Colours. 

Transmission  of  Light  by  Transparent  Bodies  ;  Refraction ;  Re- 
fraction of  the  Atmosphere  ;  Refraction  of  a  Lens  ;  Refraction 
of  the  Prism  ;  Of  the  Colours  of  Rays  of  Light ;  Of  the  Colours 
of  Bodies.  Page  211 

CONVERSATION  XVIL 

OPTICKS. 

On  the  Structure  of  the  Eye,  and  Optical  Instruments. 

Description  of  the  Eye  ;  Of  the  Image  on  the  Retina  ;  Refraction 
of  the  Humours  of  the  Eye  ;  Of  the  Use  of  Spectacles  ;  Of  the 
Single  Microscope  ;  Of  the  Double  Microscope  ;  Of  the  Solar 
Microscope  ;  Magick  Lantern  ;  Refracting  Telescope  ;  Reflect- 
ing Telescope.  Page  229. 

A  Dictionary  of  Philosophical  Terms.  Page  IMl. 


Directitm  to  the  Binder. 

The  Plates,  with  the  exception  of  the  FrontispieGC, 
which  is  to  face  the  Title  Page,  to  be  put  at  the  close 
of  the  volume,  in  their  order  of  being  numbered. 


CONVERSATION  L 


ON  GENERAL  PROPERTIES  OF  BODIES. 

Introduction ;  General  Properties  of  Bodies ;  Impentf 
trability  ;  Extension  ;  Figure ;  Divisibility  ;  Inertia ; 
Attraction;  Attraction  of  Cohesion;  Density;  Rarity  ; 
Heat ;  Attraction  of  Gravitation* 

EMILY. 

I  MUST  request  your  assistance,  my  dear  Mrs.  B.  in  a 
charge  which  I  have  lately  undertaken ;  it  is  that  of  in- 
structing my  youngest  sister,  a  task,  which  I  find  proves 
more  difficult  than  I  had  at  first  imagined.  I  can  teach 
her  the  common  routine  of  children's  lessons  tolerably 
well ;  but  she  is  such  an  inquisitive  little  creature,  that 
she  is  not  satisfied  without  an  explanation  of  every  diffi- 
culty that  occurs  to  her,  and  frequently  asks  me  questions 
which  I  am  at  a  loss  to  answer.  This  morning,  for  in- 
stance, when  I  had  explained  to  her  that  the  world  was 
round  like  a  ball,  instead  of  being  flat  as  she  had  suppos- 
ed, and  that  it  was  surrounded  by  the  air,  she  asked  me 
what  supported  it.  I  told  her  that  it  required  no  sup- 
port ;  she  then  inquired  why  it  did  not  fall  as  every 
thing  else  did.  This  I  confess  perplexed  me  ;  for  I  had 
myself  been  satisfied  with  learning  that  the  world  floated 
in  the  air,  without  considering  how  unnatural  it  was  that 
sjj  heavy  a  body,  bearing  the  weight  of  all  other  things, 
should  be  able  to  support  itself. 

Mrs.  J5.  I  make  no  doubt,  my  dear,  but  that  I  shall 
be  able  to  explain  this  difficulty  to  you ;  but  I  believe 
that  it  would  be  almost  impossible  to  render  it  intelligible 
to  the  comprehension  of  so  young  a  child  as  your  sister 
Sophia.  You,  who  are  now  in  your  thirteenth  year,  may, 
I  think,  with  great  propriety,  learn  not  only  the  cause  of 
this  particular  fact,  but  acquire  a  general  knowledge  of 
the  laws  by  which  the  natural  world  is  governed. 

Emily,  Of  all  things  it  is  what  I  should  most  like  to 
learn  ;  but  I  was  afraid  it  was  too  difficult  a  study  even  at 
my  age. 


10  GENERAL  PROPERTIES  OF  BODIES. 

Mrs,  B.  Not  whon  familiarly  explained  ;  if  you  have 
patience  to  attend,  I  will  most  willingly  give  you  all  the 
information  in  my  power.  You  may  perliaps  find  the 
f^ubject  rather  dry  at  first ;  but  if  I  succeed  in  explaining 
the  laws  of  nature,  so  as  to  make  you  understand  them,  I 
am  sure  that  you  will  derive  not  only  instruction,  but 
great  amusement  from  that  ntady. 

Emily,  I  make  no  doubt  of  it,  Mrs.  B. ;  and  pray 
begin  by  explaining  why  the  earth  requires  no  support ; 
for  that  is  the  point  which  just  now  most  strongly  excites 
my  curiosity. 

Mrs,  B,  My  dear  Emily,  if  I  am  to  attempt  to  give 
yciW  a  general  idea  of  the  laws  of  nature,  which  is  no  less 
than  to  intro^ace  you  to  a  knowledge  of  the  science  of 
natural  philosophy,  it  will  be  necessary  for  us  to  proceed 
with  some  de^rroe  of  regularity.  I  do  not  wish  to  confine 
you  to  the  systematic  order  of  a  scientific  treatise ;  but  if 
we  were  merely  to  examine  every  vague  question  that 
may  chance  to  occur,  our  progress  would  be  but  very  slow. 
Let  us,  therefore,  begin  by  taking  a  short  survey  of  the 
general  properties  of  bodies,  some  of  which  must  necessa- 
rily be  explained  before  I  can  attempt  to  make  you  under- 
stand why  the  earth  requires  no  support. 

When  I  speak  of  bodies,  I  mean  substances,  of  what- 
ever nature,  whether  solid  or  fluid ;  and  matter  is  the  ge- 
neral term  used  to  denote  the  substance,  whatever  its 
nature  be,  of  which  the  different  bodies  are  composed. 
Thus,  wood  is  the  matter  of  which  this  table  is  made ; 
water  is  the  matter  with  which  this  glass  is  filled,  &c. 

Emily,  I  am  very  glad  you  have  explained  the  mean- 
ing of  the  word  matter,  as  it  has  corrected  an  erroneous 
conception  I  had  formed  of  it ;  I  thought  that  it  was  ap- 
plicable to  solid  bodies  only. 

Mrs,  B,  There  are  certain  properties  which  appear 
to  be  common  to  all  bodies,  and  are  hence  called  the  e5- 
sential  properties  of  bodies  ;  these  are.  Impenetrability, 
Extension^  Fi^ure^  Divisibility^  Inertia,  and  Attraction, 
These  are  called  the  general  properties  of  bodies,  as  we 
do  not  suppose  any  body  to  exist  without  them. 


1.  What  is  to  be  understood  by  the  term  bodies,  as  used  in  phi- 
losophy ? 2.     Wliat  term  is  used  to  denote  substances  ? 3. 

What   properties   are    common  to   all  bodies  ? 4.     Why   are 

these  called  general  properties  of  bodies  ? 


GENERAL  PROPERTIES  OF  BODIES.  11 

By  impenetrahility ^  is  meant  the  property  which  bodies 
have  of  occupying  a  certain  space,  so  that,  where  one 
body  is,  another  cannot  be,  without  displacing  the  for- 
mer ;  for  two  bodies  cannot  exist  in  the  same  place  at  the 
same  time.  A  liquid  may  be  more  easily  removed  than 
a  solid  body  ;  yet  it  is  not  the  less  substantial,  since  it  is 
as  impossible  for  a  liquid  and  a  solid  to  occupy  the  same 
space  at  the  same  time,  as  for  two  solid  bodies  to  do  so. 
For  instance,  if  you  put  a  spoon  into  a  glass  full  of  water, 
the  water  will  flow  over  to  make  room  for  the  spoon. 

Emily.  I  understand  this  perfectly.  Liquids  are  in 
reality  as  substantial  or  as  impenetrable  as  solid  bodies, 
and  they  appear  less  so,  only  because  they  are  more  ea- 
sily displaced. 

Mrs.  B.  The  air  is  a  fluid  differing  in  its  nature  from 
liquids,  but  no  less  impenetrable.  If  I  endeavour  to  fill 
this  phial  by  plunging  it  into  this  bason  of  water,  the  air, 
you  see,  rushes  out  of  the  phial  in  bubbles,  in  order  to 
make  way  for  the  water,  for  the  air  and  the  water  cannot 
«xist  together  in  the  same  space,  any  more  than  two 
hard  bodies ;  and  if  I  reverse  this  goblet,  and  plunge  it  per- 
pendicularly into  the  water,  so  that  the  air  will  not  be  able 
to  escape,  the  water  will  no  longer  be  able  to  fill  the  goblet. 

Emily.     But  it  rises  a  considerable  way  into  the  glass. 

Mrs.  B.  Becn.use  the  water  compresses  or  squeezes 
the  air  into  a  small  space  in  the  upper  part  of  the  glass  ; 
but,  as  long  as  it  remains  there,  no  other  body  can  occu- 
py the  same  place. 

Emily.  A  difficulty  has  just  occurred  to  me,  with  re- 
gard to  the  impenetrability  of  solid  bodies ;  if  a  nail  is 
driven  into  a  piece  of  trood,  it  penetrates  it,  and  both 
the  wood  and  the  nail  occupy  the  same  space  that  the 
wood  alone  did  before. 

31rs.  B.  The  nail  penetrates  between  the  particles  of 
the  wood,  by  forcing  them  to  make  way  for  it ;  for  you 
know  that  not  a  single  atom  of  wood  can  remain  in  the 
space  which  the  nail  occupies  ;  and  if  the  wood  is  not 
increased  in  size  by  the  addition  of  the  nail,  it  is  because 
wood  is  a  porous  substance,  like  sponge,  the  particles  of 

5.     What  is  impenetrability  ? 6.     Can  liquids  occupy  the 

same  space  of  a  solid  body  ?- 7.     How  can  you  prove  that  they 

cannot  occupy  the  same  space  occupied  by  solids  ? 8.    Can  hit 

quids  and  air  occupy  the  same  space  in  the  same  time  ? 9.  How 

would  you  prove  that  they  cannot  ? 


12  GENERAL  PROPERTIES  OF  BODIES. 

which  may  be  compressed  or  squeezed  closer  together  ; 
and  it  is  thus  that  they  make  way  for  the  nail. 

We  may  now  proceed  to  the  next  general  property  of 
bodies,  extension,  A  body  which  occupies  a  certain 
space  must  necessarily  have  extension  ;  that  is  to  say, 
length,  breadth,  and  depth ;  these  are  called  the  dimen- 
sions of  extension  ;  can  you  form  an  idea  of  any  body 
without  them  ? 

Emily,  No :  certainly  I  cannot ;  though  these  dimen- 
sions must,  of  course,  vary  extremely  in  different  bodies. 
The  length,  breadth,  and  depth,  of  a  box,  or  of  a  thimble, 
are  very  different  from  those  of  a  walking-stick,  or  of  a 
hair. 

But  is  not  height  also  a  dimension  of  extension  ? 

Mrs,  B,  Height  and  depth  are  the  same  dimension, 
considered  in  different  points  of  view  ;  if  you  measure  a 
body,  or  a  space,  from  the  top  to  the  bottom,  you  call  it 
depth  ;  if  from  the  bottom  upwards,  you  call  it  height ; 
thus  the  depth  and  height  of  a  box  are,  in  fact,  the  same 
thing. 

Emily,  Very  true  ;  a  moment's  consideration  would 
have  enabled  me  to  discover  that ;  and  breadth  and 
width  are  also  the  same  dimension. 

Mrs,  B,  Yes ;  the  limits  of  extension  constitute  fi- 
gure or  shape.  You  conceive  that  a  body  having  length, 
breadth,  and  depth,  cannot  be  without  form,  either  sym- 
metrical or  irregular. 

Emily,  Undoubtedly  ;  and  this  property  admits  of  al- 
most an  infinite  variety. 

Mrs,  B,  Nature  has  assigned  regular  forms  to  her 
productions  in  general.  The  natural  form  of  mineral  sub- 
stances is  that  of  crystals,  of  which  there  is  a  great  variety. 
Many  of  them  are  very  beautiful,  and  no  less  remarkable 
by  their  transparency,  or  colour,  than  by  the  perfect 
regularity  of  their  forms,  as  may  be  seen  in  the  various 
museums  and  collections  of  natural  history.  The  vege- 
table and  animal  creation  appears  less  symmetrical,  but  is 
still  more  diversified  in  figure  than  the  mineral  kingdom. 

10.  What  is  meant  by  extension  ? 11.  What  is  the  differ- 
ence between  height  and  depth  as  applied  to  extension  .'* 12 

What  is  the  figure  of  a  body  ? 13.     What  forms  has  nature,  in 

general,  given  to  her  productions  ^•^ 14.    What  is  said  of  mine 

ral  substances  ? 15.     How  does  the  vecetable  and  animal  ere 

ation  compare  with  the  mineral  kingdom  ? 


ffENERAL  PROPERTIES  OF  BOIilES.  13 

Manufactured  substances  assume  the  various  arbitrary 
forms  which  the  art  of  man  designs  for  them  ;  and  an  in- 
finite number  of  irregular  forms  are  produced  by  frac- 
tures, and  by  the  dismemberment  of  the  parts  of  bodies. 
Emily,  Such  as  a  piece  of  broken  china  or  glass  ? 
Mrs,  B.  Or  the  fragments  of  mineral  bodies  which  are 
broken  in  being  dug  out  of  the  earth,  or  decayed  by  the 
eifect  of  torrents  and  other  causes.  The  picturesque  ef- 
fect of  rock-scenery  is  in  a  great  measure  owing  to  acci- 
iiental  irregularities  of  this  kind. 

We  may  now  proceed  to  dunsihiiitij  ;  tliat  is  to  say,  a 
susceptibility  of  being  divided  into  an  indefinite  number  of 
parts.  Take  any  small  quantity  of  matter,  a  grain  of  sand 
for  instance,  and  cut  it  into  tw^o  parts  ;  these  two  parts 
might  be  again  divided,  had  v/e  instruments  sufficiently 
fine  for  the  purpose  ;  and  if,  by  means  of  pounding,  grind- 
ing and  other  similar  methods,  we  carry  this  division  to 
the  greatest  possible  extent,  and  reduce  the  body  to  its 
finest  imaginable  particles,  yet  not  one  of  the  particles  will 
be  destroyed,  and  the  body  will  continue  to  exist,  though 
in  this  altered  state. 

The  melting  of  a  solid  body  in  a  liquid  affords  a  very 
striking  example  of  the  extreme  divisibility  of  matter  ; 
when  you  sweeten  a  cup  of  tea,  for  instance,  with  what 
minuteness  the  sugar  must  be  divided  to  be  diffused 
throughout  the  whole  of  the  liquid. 

Emily,  And  if  you  pour  a  few  drops  of  red  wine  into 
a  glass  of  water,  they  immediately  tinge  the  whole  of  the 
water,  and  must  therefore  be  diffused  throughout  it. 

Mrs,  B.  Exactly  so  ;  and  the  perfume  of  this  laven- 
der water  will  be  almost  as  instantaneously  diffused 
throughout  the  room,  if  I  take  out  the  stopper. 

Emily,  But  in  this  case  it  is  only  the  perfume  of  the  la- 
vender, and  not  the  water  itself,  that  is  diffused  in  the  room  ? 
Mrs,  B,  The  odour  or  smell  of  a  body  is  part  of  the 
body  itself,  and  is  produced  by  very  minute  particles  or 
exhalations  which  escape  from  odoriferous  bodies.  It 
would  be  impossible  that  you  should  smell  the  lavender 
water,  if  particles  of  it  did  not  come  in  actual  contact 
with  your  nose. 

16.     What  is  divisibilitv  in  natural  philosophy  ? 17.     What 

are  instances  of  practical  divisibility  of  matter  to  a  ffreat  ex- 
tent ? 18.     On  what  principle  is  it  that  we  can  smell  odorife- 

^crus  objects  ? 

2 


14  GENERAL  PROPERTIES  OF  BODIES. 

Emily,  But  when  I  smell  a  flower,  I  see  no  vapour 
rise  from  it ;  and  yet  I  can  perceive  the  smell  at  a  con- 
siderable distance. 

Mrs,  B,  You  could,  I  assure  you,  no  more  smell  a 
flower,  the  odoriferous  particles  of  which  did  not  touch 
your  nose,  than  you  could  taste  a  fruit,  the  flavoured  par- 
ticles of  which  did  not  come  in  contact  with  your  tongue. 

Emihj,  That  is  wonderful  indeed  ;  the  particles,  then, 
which  exhale  from  the  flower  and  from  the  lavender  water 
are,  I  suppose,  too  small  to  be  visible  ? 

Mrs,  B,  Certainly  :  you  may  form  some  idea  of 
their  extreme  minuteness  from  the  immense  number 
which  must  have  escaped  in  order  to  perfume  the  whole 
room  ;  and  yet  there  is  no  sensible  diminution  of  the 
liquid  in  the  phial. 

Emily,     But  the  quantity  must  really  be  diminished  ? 

Mrs,  B,  Undoubtedly  ;  and  were  you  to  leave  the 
bottle  open  a  sufl^cient  length  of  time,  the  whole  of  the 
water  would  evaporate  and  disappear.  But  though  so 
minutely  subdivided  as  to  be  imperceptible  to  any  of  our 
senses,  each  particle  would  continue  to  exist ;  for  it  is 
not  within  the  power  of  man  to  destroy  a  single  particle 
of  matter  :  nor  is  there  any  reason  to  suppose  that  in  na- 
ture an  atom  is  ever  annihilated. 

Emily.  Yet,  when  a  body  is  burnt  to  ashes,  part  of  it, 
at  least,  appears  to  be  effectually  destroyed  ?  Look  how 
small  is  the  residue  of  ashes  beneath  the  grate,  from  all 
the  coals  which  have  been  consumed  within  it. 

Mrs,  B.  That  part  of  the  coals,  which  you  sup)X)se 
to  be  destroyed,  evaporates  in  the  form  of  smoke  and  va- 
pour, whilst  the  remainder  is  reduced  to  ashes.  A  body, 
in  burning,  undergoes  no  doubt  very  remarkable  changes  ; 
it  is  generally  subdivided  ;  its  form  and  colour  altered  ; 
its  extension  increased  ;  but  the  various  parts,  into  w  hich 
it  has  been  separated  by  combustion,  continue  in  exist- 
ence, and  retain  all  the  essential  properties  of  bodies. 

Emily,  But  that  part  of  a  burnt  body  which  evapo- 
rates in  smoke  has  no  figure  ;  smoke,  it  is  true,  ascends 

19.     If  we  inhale  particles  of  odoriferous  objects,  why  cannot 

we  see  these  particles  ? 20.     If  the  particles  of  fragrant  liquid 

in  a  phial  escape  from  the  phial  in  order  to  perfume  a  room,  why 

can  we  not  see  them  epcape  ? ^21.     Is  not  the  matter,  of  which 

wood    is    composed,  destroyed    or    annihilated,   when   burnt   to 
ashes  •' 


GENERAL  PROPERTIES  OF  BODIES.  15 

in  columns  into  the  air,  but  it  is  soon  so  much  diffused  as 
to  lose  all  form  ;  it  becomes  indeed  invisible. 

Mrs.  B,  Invisible,  I  allow  ;  but  we  must  not  imagine 
that  what  we  no  longer  see  no  longer  exists.  Were  every 
particle  of  matter  that  becomes  invisible  annihilated,  the 
world  itself  would  in  the  course  of  time  be  destroyed. 
The  particles  of  smoke,  when  diffused  in  the  air,  continue 
still  to  be  particles  of  matter,  as  well  as  when  more  closely 
united  in  the  form  of  coals  :  they  are  really  as  substantial 
in  the  one  state  as  in  the  other,  and  equally  so  when  by 
their  extreme  subdivision  they  become  invisible.  No 
particle  of  matter  is  ever  destroyed  :  this  is  a  principle 
you  must  constantly  remember.  Every  thing  in  nature 
decays  and  corrupts  in  the  lapse  of  time.  We  die,  and 
our  bodies  moulder  to  dust ;  but  not  a  single  atom  of 
them  is  lost ;  they  serve  to  nourish  the  earth,  Avhence, 
while  living,  they  drew  their  support.* 

The  next  essential  property  of  matter  is  called  inertia ; 
this  word  expresses  the  resistance  which  inactive  matter 
makes  to  a  change  of  state.  Bodies  appear  to  be  equally 
incapable  of  changing  their  actual  state,  whether  it  be 
of  motion  or  of  rest.  You  know  that  it  requires  force 
to  put  a  body  which  is  at  rest  in  motion  ;  an  exertion 
of  strength  is  also  requisite  to  stop  a  body  which  is 
already  in  motion.  The  resistance  of  the  body  to  a 
change  of  state,  in  either  case,  is  called  its  inertia, 

Emily,  In  playing  at  base-ball  I  am  obliged  to  use 
all  my  strength  to  give  a  rapid  motion  to  the  ball ;  and 
when  I  have  to  catch  it,  I  am  sure  I  feel  the  resistance 


'^  As  a  further  illustration  of  the  great  practical  divisi- 
bility of  matter,  it  may  be  added,  that  a  single  grain  of  gold  may 
be  hammered  by  a  gold-beater  until  it  will  cover  fifty  square 
inches.  Each  square  inch  may  then  be  divided  into  two  hundred 
strips,  and  each  strip  into  two  hundred  parts,  which  may  be  seen 
with  the  naked  eye  ;  consequently,  a  square  inch  contains  forty 
thousand  visible  parts,  which  multiplied  by  50,  the  number  of 
square  inches  which  a  grain  of  gold  will  make,  give  two  million 
parts,  which  may  be  seen  with  the  naked  eye. — It  has  also  been 
calculated,  that  sixteen  ounces  of  gold,  which,  in  the  form  of  a 
cube,  would  not  measure  one  inch  and  a  quarter  in  its  side,  will 
completely  gild  a  quantity  of  silver  wire  sufficient  to  surround 
the  globe. 

22.     Is  it  a  principle  in  natural  philosophy  that  no  particle  of 

matter  can  be  destroyed  ? 23.     What  is  meant  by  the  term 

inertia  ^ ^24.     What  instances  of  great  'practice-  divisibility  of 

matter  are  given  in  tlie  note  ? 


16  GENERAL  PROPERTIES  OF  BODIES. 

it  makes  to  being  stopped.     But  if  I  did  not  catch  it,  it 
would  soon  fall  to  the  ground  and  stop  of  itself. 

Mrs,  B.  Inert  matter  is  as  incapable  of  stopping  of  it- 
self, as  it  is  of  putting  itself  in  motion:  when  the  ball  ceases 
to  move,  therefore,  it  must  be  stopped  by  some  other  cause 
or  power  ;  but  as  it  is  one  with  which  you  are  yet  un- 
acquainted,  we   cannot  at  present  investigate  its  effects. 

The  last  property  which  appears  to  be  common  to  all 
bodies  is  attraction.  All  bodies  consist  of  infinitely  small 
particles  of  matter,  each  of  which  possesses  the  power  of 
attracting  or  drawing  towards  it,  and  uniting  with  any 
other  particle  sufficiently  near  to  be  within  the  influence 
of  its  attraction  ;  but  in  minute  particles  this  power  ex- 
tends to  so  very  small  a  distance  around  them  that  its 
effect  is  not  sensible,  unless  they  are  (or  at  least  appear 
to  be)  in  contact ;  it  then  makes  them  stick  or  adhere 
together,  and  is  hence  called  the  attraction  of  cohesion. 
Without  this  power,  solid  bodies  would  fall  in  pieces,  or 
rather  crumble  to  atoms. 

Emily.  I  am  so  much  accustomed  to  see  bodies  firm  and 
solid,  that  it  never  occurred  to  me  that  any  power  w^as 
requisite  to  unite  the  particles  of  which  they  are  composed. 
But  the  attraction  of  cohesion  does  not,  I  suppose,  exist 
in  liquids  ;  for  the  particles  of  liquids  do  not  remain  to- 
gether so  as  to  form  a  body,  unless  confined  in  a  vessel  ? 

Mrs,  B,  I  beg  your  pardon  ;  it  is  the  attraction  of 
cohesion  which  holds  this  drop  of  water  suspended  at  the 
end  of  my  finger,  and  keeps  the  minute  watery  particles 
of  which  it  is  composed  united.  But  as  this  power  is 
stronger  in  proportion  as  the  particles  of  bodies  are  more 
closely  united,  the  cohesive  attraction  of  solid  bodies 
is  much  greater  than  that  of  fluids.  The  thinner  and 
lighter  a  fluid  is,  the  less  is  the  cohesive  attraction  of 
its  particles,  because  they  are  further  apart ;  and  in  elastic 
fluids,  such  as  air,  there  is  no  cohesive  attraction  among 
the  particles. 

25.     What  would  be  the  consequence,  if  a  body   were  put  in 

motion  and  no  resistance  should  be  offered  ? 26.     What  is  the 

property  common  to  all  bodies  ? — ^27.     Of  what  do  all  bodies 

consist? 28.     What  is  the  power  called   which   binds   thew^ 

small   particles  together  ? 29.      What    would  be  the   conse- 
quence if  the  power  of  cohesive  attraction  were  destroyed  ? 

30.     Does  the  power  of  cohesion  exist  also   iil  liquids .''- 31. 

How  would  3^ou  prove  that  it  exists  jn  liqui^i>  ? 32.     Why  are 

^ome  bodies  liard  and  others  soft  .'* 


GENERAL  PROPERTIES  OP  BOI>IES.  17 

Emily,  That  is  very  fortunate  ;  for  it  would  be  im- 
possible to  breathe  the  air  in  a  solid  mass  ;  or  even  in 
a  liquid  state.  But  is  the  air  a  body  of  the  same  nature 
as  other  bodies  ? 

Mrs,  B,     Undoubtedly,  in  all  essential  properties. 

Emily,  Yet  you  say  that  it  does  not  possess  one  of 
the  general  properties  of  bodies — cohesive  attraction  ? 

Mrs,  B,  The  particles  of  air  are  not  destitute  of  the 
power  of  attraction,  but  they  are  too  far  distant  from  each 
other  to  be  influenced  by  it ;  and  the  utmost  efforts  of 
human  art  have  proved  ineffectual  in  the  attempt  to  com- 
press them,  so  as  to  bring  them  within  the  sphere  of  each 
other's  attraction,  and  make  them  cohere. 

Emily.  If  so,  how  is  it  possible  to  prove  that  they  are 
endowed  with  this  power  ? 

Mrs,  B,  The  air  is  formed  of  particles  precisely  of 
the  same  nature  as  those  which  enter  into  the  composi- 
tion of  liquid  and  solid  bodies,  in  which  state  we  have  a 
proof  of  their  attraction. 

Emily.  It  is  then,  I  suppose,  owing  to  the  different 
degrees  of  attraction  of  different  substances,  that  they  are 
hard  or  soft ;  and  that  liquids  are  thick  or  thin  ? 

Mrs,  B.  Yes  ;  but  you  would  express  your  meaning 
better  by  the  term  density,  which  denotes  the  degree  of 
closeness  and  compactness  of  the  particles  of  a  body : 
thus  you  may  say,  both  of  solids,  and  of  liquids,  that  the 
stronger  the  cohesive  attraction  the  greater  is  the  den- 
sity of  the  body.  In  philosophical  language,  density 
is  said  to  be  that  property  of  bodies  by  which  they  ccm- 
tain  a  certain  quantity  of  matter,  under  a  certain  bulk  .or 
magnitude.  Rarity  is  the  contrary  of  density ;  it  denotes 
the  thinness  and  subtlety  of  bodies  :  thus  you  would  say 
that  mercury  or  quicksilver  was  a  very  dense  fluid  ; 
ether,  a  very  rare  one,  &c. 

Caroline,  But  how  are  we  to  judge  of  the  quantity  of 
matter  contained  in  a  certain  bulk  ? 


33.  Does  the  attraction  of  cohesion  exist  in  the  air  ? — 34.  But 
are  the  particles  of  the  air  actually  under  the  influence  of  this 

attraction.^ 35.     Why  are   they  not,  if  attraction  belong  to 

them  .'' 36.     How  do  we  know  that  attraction  does  belon;^  to 

the  air  if  no  influence  is  exerted  upon  it  ? 37.     What  is  meant 

by  the  term  density  ? 38.    What  is  meant  by  the  term  rarity  ^ 

2  * 


18  GENERAL  PROPERTIES  OF  DOrHEr-. 

Mrs,  15.  By  the  weight :  under  the  same  bulk  bodies 
are  said  to  be  dense  in  proportion  as  they  are  heavy .. 

Emily,  Then  we  may  say  tliat  metals  are  dense  bodies, 
wood  comparatively  a  rare  one,  ^c.  But,  Mrs,  B.,  when 
the  particles  of  a  body  are  so  near  as  to  attract  each  other, 
the  effect  of  this  power  must  increase  as  they  are  brought 
by  it  closer  together  ;  so  that  one  would  suppose  that  the 
body  would  gradually  augment  in  density,  till  it  was  im- 
possible for  its  particles  to  be  more  closely  united.  Now  we 
know  that  this  is  not  the  case  ;  for  soft  bodies,  such  as  cork, 
sponge,  or  butter,  never  become,  in  consequence  of  the  in- 
creasing attraction  of  their  particles,  as  hard  as  iron  ? 

Mrs,  B,  In  such  bodies  as  cork  and  sponge,  the  parti- 
cles which  come  in  contact  are  so  few  as  to  produce  but  a 
slight  degree  of  cohesion  ;  they  are  porous  bodies,  which, 
owing  to  the  peculiar  arrangement  of  their  particles,  abound 
with  interstices  which  separate  the  particles ;  and  these 
vacancies  are  filled  with  air,  the  spring  or  elasticity  of 
which  prevents  the  closer  union  of  the  parts.  But  tliere  is 
another  fluid  much  more  subtle  than  air,  which  pervades  all 
bodies,  this  is  heat.  Heat  insinuates  itself  more  or  less  be- 
tween the  particles  of  all  bodies,  and  forces  them  asunder ; 
you  may  therefore  consider  heat  and  the  attraction  of  co- 
hesion, as  constantly  acting  in  opposition  to  each  other. 

Emily,  The  one  endeavouring  to  rend  a  body  to 
pieces,  the  other  to  keep  its  parts  firmly  united. 

Mrs,  B,  And  it  is  this  struggle  between  the  contend- 
ing forces  of  heat  and  attraction,  which  prevents  the  ex- 
treme degree  of  density  which  would  result  from  the  sole 
influence  of  the  attraction  of  cohesion. 

Emily,  The  more  a  body  is  heated  then,  the  more  its 
particles  will  be  separated. 

Mrs,  B,  Certainly ;  we  find  that  bodies  swell  or  dilate 
by  heat :  this  effect  is  very  sensible  in  butter,  for  instance, 
which  expands  by  the  application  of  heat :  till  at  length 

39.     How  are  we  to  judge  of  the  quantity  of  matter  in  bodies? 

40.  In  what  proportion  are  bodies  dense  of  the  same  bulk  ? 

41.     What  bodies  are  usually  said  to  be  dense  ? 42.     What 

ones  are  said  to  be  rare  ? 43.     Why   are  not  sponge  and  cork 

and  other  similar  substances  hard,  since  their  particles  come  in 

contact  ? 44.     What  fluid  is  named  more  subtle  than  air  .' 

45.     What  effect  has  heat  on  bodies  ? 40.     What  two  forces 

are  said  to  act  always  on  bodies  in  opposition  to  each  otlier  .'* • 

47.     In  what  cases  may  we   see  the  effect  of  heat  in  the  ex  par?  - 
eion  of  bodies,  or  in  the  separation  of  their  particles  ? 


GENERAL  PROPERTIES  OF  BODIES.  19 

the  attraction  of  cohesion  is  so  far  diminished  that  the  par^ 
tides  separate,  and  the  butter  becomes  liquid.  A 'similar 
effect  is  produced  by'  heat  on  metals,  and  all  bodi^  sus- 
ceptible of  being  melted.  Liquids,  you  know,  are  made 
to  boil  by  the  application  of  heat :  the  attraction  of  cohe- 
sion then  yields  entirely  to  the  expansive j|)ovver  ;  the 
particles  are  totally  separated  and  converted  into  steam 
or  vapour.  But  the  agency  of  heat  is  in  no  body  more  seiv 
sible  than  in  air,  which  dilates  and  contracts  by  its  in- 
crease or  diminution  in  a  very  remarkable  degree.* 

Emily,  The  effects  of  heat  appear  to  be  one  of  the 
most  interesting  parts  of  natural  philosophy. 

Mrs,  B,  That  is  true  ;  but  heat  is  so  intimately  con- 
nected with  chemistry,  that  you  must  allow  me  to  defer 
the  investigation  of  its  properties  till  you  become  ac- 
quainted with  that  science. 

To  return  to  its  antagonist,  the  attraction  of  cohesion  ; 
it  is  this  power  which  restores  to  vapour  its  liquid  form, 
which  unites  it  into  drops  when  it  falls  to  the  earth  in  a 
shower  of  rain,  which  gathers  the  dew  into  brilliant  gems 
pn  the  blades  of  grass. 

Einily,  And  I  have  often  observed  that  after  a  shower, 
the  water  collects  into  large  drops  on  the  leaves  of 
plants  ;  but  I  cannot  say  that  I  perfectly  understand  how 
the  attraction  of  cohesion  produces  this  effect. 

3Irs,  B,  Rain  does  not  fall  from  the  clouds  in  the  form 
of  drops,  but  in  that  of  mist  or  rapour,  which  is  composed 
of  very  small  watery  particles  ;  these  in  their  descent, 
mutually  attract  each  other,  and  those  that  are  sufficient- 
ly near  in  consequence  unite  and  form  a  drop,  and  thus 

*  The  expansive  power  of  heat  produces  some  of  the  most  in- 
teresting phenomena  in  nature.  The  boiling  of  liquids,  is  the  im- 
mediate result  of  this  power  ;  and  the  operation,  although  simple, 
is  peculiarly  worthy  of  notice.  As  the  numerous  particles  become 
expanded  or  rarified,  they  are  continually  rising  to,  and  escaping 
from  the  surface,  which  occasions  an  agitation  of  the  liquid,  pro- 
portioned, in  its  violence,  to  the  degree  of  heat  operating  on 
it. — And  on  exposing  our  hands  or  other  limbs  to  the  fire,  the 
internal  fluid  becomes  expanded,  which  causes  them  to  appear 
swollen ;  whereas,  when  exposed  to  the  cold,  the  abstraction  af 
the  heat  causes  them  to  be  compressed. 

AQ.  How  arc  liquids  made  to  boil  by  heat  ;  or  hoio  is  the  mo- 
tion or  acritation  of  boiling  liquids  produced  ? 49.     Why  are 

our  hands  and  fingers  swollen  or  larger  on  being  held  near  the 

fire,  than  ichen  exposed  to  the  cold  ? 50.     In  what  state  does 

rain  fall  from  the  clouds^? 51,       What  collects  this  mist  or 

vapour  into  drops  ? 


2D  GENERAL  PROPERTIES  OF  BODIES. 

the  mist  is  transformed  into  a  shower.  The  dew  also  was 
originally  in  a  state  of  vapour,  but  is,  by  the  mutual  at- 
traction of  the  particles,  formed  into  small  globules  on  the 
blades  of  grass  :  in  a  similar  manner  the  rain  upon  the 
leaf  collects  into  large  drops,  which,  when  they  become  too 
heavy  for  the  leaf  to  support,  fall  to  the  ground. 

Emily.  All  this  is  wonderfully  curious !  I  am  almost 
bewildered  with  surprise  aiKl  admiration  at  the  number 
of  new  ideas  I  have  already  acquired. 

Mrs.  B,  Every  step  that  you  advance  in  the  pursuit 
of  natural  science,  will  fill  your  mind  with  admiration  and 
gratitude  towards  its  Divine  Author.  In  the  study  of 
natural  philosophy,  we  must  consider  ourselves  as  read- 
ing the  book  of  nature,  in  which  the  bountiful  goodness 
and  wisdom  of  God  is  revealed  to  all  mankind ;  no  study 
can  then  tend  more  to  purify  the  heart,  and  raise  it  to  a 
religious  contemplation  of  the  Divine  perfections. 

There  is  another  curious  eftect  of  the  attraction  of  co- 
hesion which  I  must  point  out  to  you.  It  enables  liquids 
to  rise  above  their  level  in  capillary  tubes  ;  these  are 
tubes,  the  bores  of  which  are  so  extremely  small  that  li- 
quids ascend  within  them,  from  the  cohesive  attraction 
between  the  particles  of  the  liquid  and  the  interiour  sur- 
face of  the  tube.  Do  you  perceive  the  water  rising  above 
its  level  in  this  small  glass  tube,  which  I  have  immersed 
in  a  goblet  full  of  water  ? 

Emily,  Oh  yes ;  I  see  it  slowly  creeping  up  the  tube» 
but  now  it  is  stationary  ;  will  it  rise  no  higher  ? 

Mrs.  B,  No ;  because  the  cohesive  attraction  be- 
tween the  water  and  the  internal  surface  of  the  tube  is 
now  balanced  by  the  weight  of  the  water  within  it :  if  the 
bore  of  the  tube  were  narrower,  the  water  would  rise 
higher  ;  and  if  you  immerse  several  tubes  of  bores  of  dif- 
ferent sizes,  you  will  see  it  rise  to  different  heights  in 
■each  of  them.  In  making  this  experiment,  you  should 
colour  the  water  with  a  little  red  wine,  in  order  to  render 
the  effect  more  obvious. 

All  porous  substances,  such  as  sponge,  bread,  linen, 
&c.  may  be  considered  as  collections  of  capillary  tubes  : 
if  you  dip  one  end  of  a  lump  of  sugar  into  water,  the 

52.     What  causes  the  dew  on  leaves  and  blades  of  grass  to 

collect  into  drops  ? 53.  Why  will  liquids  rise  above  their  level 

in  capillary  tubes  ? 54.     On  what  principle  -do  sponge,  and 

other  porous  subs^^mces  absorb  liquids  ? 


GENERAL  PROPERTIES  OF  BODIES.  2^1 

water  will  rise  in  it ;  and  wet  it  considerably  above  the 
surface  of  that  into  which  you  dip  it. 

Emily,  In  making  tea  I  have  often  observed  that 
effect  without  being  able  to  account  for  it. 

Mrs,  B,  Now  that  you  are  acquainted  with  the  at- 
traction of  cohesion,  I  must  endeavour  to  explain  to  you 
that  of  Gravitation,  which  is  a  modification  g^  the  same 
power ;  the  first  is  perceptible  only  in  very  minute  parti- 
cles, and  at  very  small  distances ;  the  other  acts  on  the 
largest  bodies,  and  extends  to  immense  distances. 

Emily,  You  astonish  me  :  surely  you  do  not  mean  to 
say  that  large  bodies  attract  each  other. 

Mrs,  B,  Indeed  I  do  :  let  us  take,  for  example,  one 
of  the  largest  bodies  in  nature,  and  observe  whether  it 
does  not  attract  other  bodies.  What  is  it  that  occasions 
the  fall  of  this  book,  when  I  no  longer  support  it  ? 

Emily,  Can  it  be  the  attraction  of  the  earth  ?  I 
thought  that  all  bodies  had  a  natural  tendency  to  fall. 

Mrs,  B,  They  have  a  natural  tendency  to  fall,  it  is 
true  ;  but  that  tendency  is  produced  entirely  by  the  at- 
traction of  the  earth ;  the  earth  being  so  much  larger 
than  any  body,  on  its  surface,  forces  every  body,  which 
is  not  supported,  to  fall  upon  it. 

Emily,  If  the  tendency  which  bodies  have  to  fall 
results  from  the  earth's  attractive  power,  the  earth  itself 
can  have  no  such  tendency,  since  it  cannot  attract  itself, 
and  therefore  it  requires  no  support  to  prevent  it  from 
falling.  Yet  the  idea  that  bodies  do  not  fall  of  their  own 
accord,  but  that  they  are  drawn  towards  the  earth  by  its 
attraction,  is  so  new  and  strange  to  me,  that  I  know  not 
how  to  reconcile  myself  to  it. 

Mrs,  B,  When  you  are  accustomed  to  consider  the 
fall  of  bodies  as  depending  on  this  cause,  it  will  appear 
to  you  as  natural,  and  surely  much  more  satisfactory,  than 
if  the  cause  of  their  tendency  to  fall  were  totally  unknown. 
Thus  you  understand,  that  all  matter  is  attractive,  from 
the  smallest  particle  to  the  largest  mass  ;  and  that  bodies 
attract  each  other  with  a  force  proportional  to  the  quan- 
tity  of  matter  they  contain. 

Emily,  I  do  not  perceive  any  difference  between  the 
attraction  of  cohesion  and  that  of  gravitation  :  is  it  not  be- 

55.     What  is  the  difference  between  cohesive  attraction  and 

gravitation  ? 56.     What  causes   bodies  to  fall  to  the  earth  ? 

57.     In  what  proportion  do  bodies  gravitate  towards  or  at 

^ract  each  other  ? 


22  GENERAL  PROPERTIES  OF  BODIES. 

cause  every  particle  of  matter  is  endowed  with  an  attrac- 
tive power,  that  large  bodies,  consisting  of  a  great  num- 
ber of  particles,  are  so  strongly  attractive  ? 

Mrs.  B.  True.  There  is,  however,  this  difference 
between  the  attraction  of  particles  and  that  of  masses,  that 
the  former  is  stronger  than  the  latter,  in  proportion  to  the 
quantity  of  matter.  Of  this  you  have  an  instance  in  the 
attraction  of  capillary  tubes,  in  whicli  liquids  ascend  by 
the  attraction  of  cohesion,  in  opposition  to  that  of  gravity. 
It  is  on  this  account  that  it  is  necessary  that  the  bore  of 
the  tube  should  be  extremely  small ;  for  if  the  column  of 
water  within  the  tube  is  not  very  minute,  the  attraction 
would  not  be  able  either  to  raise  or  support  its  weight,  in 
opposition  to  that  of  gravity. 

You  may  observe,  also,  that  all  solid  bodies  are  enabled 
by  the  force  of  the  cohesive  attraction  of  their  particles 
to  resist  that  of  gravity,  which  would  otherwise  disunite 
them,  and  bring  them  to  a  level  with  the  ground,  as  it 
does,  in  the  case  of  liquids,  the  cohesive  attraction  of  which 
is  not  sufficient  to  enable  it  to  resist  the  power  of  gravity.* 

*  The  power  of  gravitation  is  greatest  at  the  surface  of  the 
earth,  whence  it  decreases  both  upwards  and  downwards ;  but 
not  in  the  same  proportion.  The  force  of  gravity  upwards  is  as 
the  square  of  the  distance  from  the  centre.  That  is,  gravity  at 
the  surface  of  the  earth,  which  is  about  4000  miles  from  the  cen- 
tre, is  four  times  more  powerful  than  it  would  be  at  double  that 
distance,  or  8000  miles  from  the  centre.  Gravity  arid  weight  may 
be  taken,  in  particular  circumstances,  as  synonymous  terms.  We 
«ay,  a  piece  of  lead  weighs  a  pound,  or  sixteen  ounces  ;  but  if  by 
any  means  it  could  be  carried  4000  miles  above  the  surface  of  the 
earth,  it  would  weigh  only  one  fourth  of  a  pound,  or  four  ounces  ; 
and  if  it  could  be  transported  to  8000  miles  above  the  earth, 
which  is  three  times  the  distance  from  the  centre  that  the  surface 
is,  it  would  weigh  only  one  ninth  of  a  pound,  or  something  less 
than  two  ounces. 

And  it  is  demonstrated,  that  the  force  of  gravity  downwards  de- 
creases, as  the  distance  from  the  surface  increases,  so  that  at  one 
half  the  distance  from  the  centre  to  the  surface,  the  same  weight- 


58.     What  example  is  given  to  show  that  cohesive  attraction  is 

stronger  than  gravitation  ? 59.     Why  must  the  bore  of  capil- 

iary  tubes  be  exceedingly  small  for  water  to  rise  in  them  .** 

GO.     What  would  be  the  effect  of  gravitation  on  bodies,  were  it  not 

for  cohesive  attraction  ? 61 .     Where  is  the  power  of  gravity 

greatest  ? G2.  In  what  proportion  does  gravity  decrease  from 

the  surface  of  the  earth  upwards  9        63.     in  what  proportion 
does  it  decrease  doionwards  f 


GENERAL  PROPERTIES  OF  BODIES.  23 

Emily.  And  some  solid  bodies  appear  to  be  of  this 
nature,  as  sand  and  powder  for  instance  :  there  is  no  at- 
traction of  cohesion  between  their  particles  1 

Mrs.  B,  Every  grain  of  powder  or  sand  is  composed 
of  a  great  number  of  other  more  minute  particles,  firmly 
united  by  the  attraction  of  cohesion  ;  but  amongst  the 
separate  grains  there  is  no  sensible  attraction,  because 
they  are  not  in  sufficiently  close  contact. 

E?niL     Yet  they  actually  touch  each  other  ? 

Mrs.  B.  Tiie  surface  of  bodies  is  in  general  so  rough 
and  uneven,  that  when  in  actual  contact,  they  touch  each 
other  only  by  a  few  points.  Thus,  if  I  lay  upon  the  table 
this  book,  the  binding  of  which  appears  perfectly  smooth; 
yet  so  few  of  the  particles  of  its  under  surface  come  in 
contact  with  the  table,  that  no  sensible  degree  of  cohesive 
attraction  takes  place  ;  for  you  see,  that  it  does  not  stick, 
or  cohere  to  the  table,  and  I  find  no  difficulty  in  lifting 
it  off. 

It  is  only  when  surfaces  perfectly  flat  and  well  polished 
are  placed  in  contact,  that  the  particles  approach  in  suffi- 
cient number,  and  closely  enough,  to  produce  a  sensible 
degree  of  cohesive  attraction.  Here  are  two  hemispheres 
of  polished  metal,  I  press  their  flat  surfaces  together,  hav- 
ing previously  interposed  a  few  drops  of  oil,  to  fill  up 
every  little  porous  vacancy.     Now  try  to  separate  them. 

already  described  would  weigh  only  one  half  of  a  pound,  and  so 
on — Thus,  a  piece  of  metal  weighing,  on  the  surface  of  the  earth, 
one  pound,  will 

At  the  centre  weigh       -     -     -     0 

1000  miles  from  the  centre,     1-4  pound. 

2000 1-2 

3000 3-4 

4000 1 

8000 1-4 

12,000 1-9 

And  at  the  distance  of  the  moon  from  the  earth  which  is 
240,000  miles,  it  would  weigh  only  the  3,  GOOth  part  of  a  pound, 
because  the  distance  is  60  times  further  from  the  centre  of  the 
earth  than  the  surface. 

64.  If  a  hodif  weigh  one  pound  at  the  surface  of  the  earthy 
what  will  he  its'iceight  at  the  centre — at  1000— ai  2000— ai  3000 
-^at  4000— rti  BOOO^anrf  at  12,000  miles  from  the  centre  of  it  ? 

65.  What  is  the  reason  that  cohesive  attraction  does  not  ope- 
rate on  different  bodies  brought  into  contact,  as  well  as  on  the 

particles  of  the  same  body  ? ^^-     When  will  the  surfaces  of 

different  bodies  adhere  to  each  other  by  the  force  of  cohesivQ 
attraction  .- 


24  ON  THE  ATTRACTION  OF  GRAVITY. 

Emily,  It  requires  an  effort  beyond  my  strengtli, 
though  there  are  handles  for  the  purpose  of  pulling  them 
asunder.  Is  the  firm  adhesion  of  the  two  hemispheres, 
merely  owing  to  the  attraction  of  cohesion  ? 

Mrs,  B,  There  is  no  force  more  powerful,  since  it  is 
by  this  tnat  the  particles  of  the  hardest  bodies  are  held 
together.  It  would  require  a  weight  of  several  pounds, 
to  separate  these  hemispheres. 

Emily,  In  making  a  kaleidoscope,  I  recollect  that  the 
two  plates  of  glass,  w  hich  were  to  serve  as  mirrors,  stuck 
so  fast  together,  that  I  imagined  some  of  the  gum  I  had 
been  using  had  by  chance  been  interposed  between  them ; 
but  now  I  make  no  doubt  but  that  it  was  their  own  natu- 
ral cohesive  attraction  which  produced  this  effect. 

Mrs,  B,  Very  probably  it  was  so  ;  for  plate-glass  has 
an  extremely  smooth,  flat  surface,  admitting  of  the  con- 
tact of  a  great  number  of  particles,  between  two  plates, 
laid  one  over  the  other. 

Emily,  But,  Mrs.  B.  the  cohesive  attraction  of  some 
bodies  is  much  greater  than  that  of  others  ;  thus,  glue, 
gum,  and  paste,  cohere  with  singular  tenacity. 

Mrs,  B,  That  is  owing  to  the  peculiar  chemical  pro- 
perties of  those  bodies,  independently  of  their  cohesive  at- 
traction. 

There  are  some  other  kinds  of  modifications  of  attrac- 
tion peculiar  to  certain  bodies  ;  namely,  that  of  magnet- 
ism, and  of  electricity  ;  but  we  shall  confine  our  attention 
merely  to  the  attraction  of  cohesion  and  of  gravity  ;  the 
examination  of  the  latter  we  shall  resume  at  our  next 
meeting. 


CONVERSATION  IL 

ON  THE  ATTRACTION  OF  GRAVITY. 

Attraction  of  Gravitation^  continued ;  Of  Weight ;  Of 
the  Fall  of  Bodies ;  Of  the  Resistance  of  the  Air ;  Of 
the  Ascent  of  Light  Bodies, 

EMILY. 

I  HAVE  related  to  my  sister  Caroline  all  that  you  have 
taught  me  of  natural  philosophy,  and  she  has  been  so 
much  delighted  by  it,  that  she  hopes  you  will  have  the 
goodness  to  admit  her  to  your  lessons. 


ON  THE  ATTRACTION  OF  GRAVITY.  25 

Mrs,  B.  Very  willingly  ;  but  I  did  not  think  you  had 
any  taste  for  studies  of  this  nature,  Caroline  ? 

Caroline.  I  confess,  Mrs.  B.,  that  hitherto  I  had  form- 
ed no  very  agreeable  idea,  either  of  philosophy,  or  philo- 
sophers ;  but  what  Emily  has  told  me,  has  excited  my  curi- 
osity so  much,  that  I  shall  be  highly  pleased  if  you  will 
allow  me  to  become  one  of  your  pupils. 

Mrs,  B,  I  fear  that  I  shall  not  find  you  so  tractable  a 
scholar  as  Emily  ;  I  know  that  you  are  much  biassed  in 
favour  of  your  own  opinions. 

Caroline,  Then  you  will  have  the  greater  merit  in  re- 
forming them,  Mrs.  B.  ;  and  after  all  the  wonders  that 
Emily  has  related  to  me,  I  think  I  stand  but  little  chance 
against  you  and  your  attractions. 

Mrs.  B,  You  will,  I  doubt  not,  advance  a  number  of 
objections  ;  but  these  I  shall  willingly  admit,  as  they  will 
be  a  means  of  elucidating  the  subject.  Emily,  do  you 
recollect  the  names  of  the  general  properties  of  bodies  ? 

Emily,  Impenetrability,  extension,  figure,  divisibility, 
inertia,  and  attraction. 

Mrs,  B,  Very  well.  You  must  remember  that  these 
are  properties  common  to  all  bodies,  and  of  which  they 
cannot  be  deprived  ;  all  other  properties  of  bodies  are 
called  accidental,  because  they  depend  on  the  relation  or 
connexion  of  one  body  to  another. 

Caroline,  Yet  surely,  Mrs.  B.,  there  are  other  proper- 
ties which  are  essential  to  bodies,  besides  those  you  have 
enumerated.  Colour  and  weight,  for  instance,  are  com- 
mon to  all  bodies,  and  do  not  arise  from  their  connexion 
with  each  other,  but  exist  in  the  bodies  themselves ;  these, 
therefore,  cannot  be  accidental  qualities. 

Mrs.  B,  I  beg  your  pardon  ;  these  properties  do  not 
exist  in  bodies  independently  of  their  connexion  with 
other  bodies. 

Caroline,  What !  have  bodies  no  weight  ?  Does  not 
this  table  weigh  heavier  than  this  book  ;  and,  if  one  thing 
weighs  heavier  than  another,  must  there  not  be  such  a 
thing  as  weight  ? 

Mrs,  B,  No  doubt :  but  this  property  does  not  appear 
to  be  essential  to  bodies  ;  it  depends  upon  their  connex- 

67.     What  were  the  names  of  the  common  or  general  properties 

of  bodies  given  in  the  first  Conversation  ? 63.     What  are  called 

the  accidental  properties  of  bodies  ? 69.    Are  colour  and  weight 

common  or  accidental  properties  ?  '  '- 

3 


26  ON  THE  ATTRACTION  OF  GRAVITY. 

ion  with  each  other.  Weight  is  an  effect  of  the  power 
of  attraction,  without  which  the  table  and  the  book  would 
have  no  weight  whatever. 

Eniihj.  I  think  I  understand  you  ;  is  it  not  the  at- 
traction of  gravity,  which  makes  bodies  heavy  ? 

Mrs,  i?.  You  are  right.  I  told  you  that  the  attrac- 
tion of  gravity  was  proportioned  to  the  quantity  of  matter 
which  bodies  contained  :  now  the  earth  consisting  of  a 
much  greater  quantity  of  matter  than  any  body  upon  its 
surface,  the  force  of  its  attraction  must  necessarily  be 
greatest,  and  must  draw  every  thing  towards  it ;  in  con- 
sequence of  which,  bodies  that  are  unsupported  fall  to  the 
ground,  whilst  those  that  are  supported  press  upon  the 
object  which  prevents  their  fall,  with  a  weight  equal  to 
the  force  with  which  they  gravitate  towards  the  earth. 

Caroline,  The  same  cause  then  which  occasions  the 
fall  of  bodies  produces  also  their  weight.  It  was  very 
dull  in  me  not  to  understand  this  before,  as  it  is  the  na- 
tural and  necessary  consequence  of  attraction  ;  but  the 
idea  that  bodies  were  not  really  heavy  of  themselves  ap- 
peared to  me  quite  incomprehensible.  But,  Mrs.  B.,  if 
attraction  is  a  property  essential  to  matter,  weight  must 
be  so  likewise  ;  for  how  can  one  exist  without  the  other  ? 

Mrs,  B,  Suppose  there  were  but  one  body  existing  in 
universal  space,  what  would  its  weight  be  I 

Caroline,  That  would  depend  upon  its  size ;  or,  more 
accurately  speaking,  upon  the  quantity  of  matter  it  con- 
tained. 

Emily,  No,  no  ;  the  body  w^ould  have  no  weight, 
whatever  were  its  size  ;  because  nothing  would  attract  it. 
Am  I  not  right,  Mrs.  B.? 

Mrs,  B,  You  are  :  you  must  allow,  therefore,  that  it 
would  be  possible  for  attraction  to  exist  without  weight ; 
for  each  of  the  particles  of  which  the  body  was  composed, 
would  possess  the  power  of  attraction  ;  but  they  could 
exert  it  only  amongst  themselves  ;  the  whole  mass,  hav- 
ing nothing  to  attract,  or  to  be  attracted  by,  w^ould  have 
no  weight. 

Caroline,  I  am  now  well  satisfied  that  weight  is  not 
essential  to  the  existence  of  bodies  ;  but  what  have  you 

70.     What  is  weight,  or  of  what  is  it  th'^  effect? 71.     If 

there  were  but  one  body  in  the  universe,  would  there  be  any  such 

thing  as  weight  t Tl.     Can  cohesive  attraction  exist   where 

there  is  no  weight  ? 


ON  THE  ATTRACTION  OF  GRAVlTy.  27 

to  object  to  colours,  Mrs.  B.  ?  You  will  not,  I  think,  deny 
that  they  really  exist  in  the  bodies  themselves. 

3Irs,  B,  When  we  come  to  treat  of  the  subject  of  co- 
lours, I  trust  that  I  shall  be  able  to  convince  you,  that  co- 
lours are  likewise  accidental  qualities,  quite  distinct  from 
the  bodies  to  which  they  appear  to  belong. 

Caroline,  Oh  do  pray  explain  it  to  us  now,  I  am  so 
very  curious  to  know  how  that  is  possible. 

Mrs.  B.  Unless  we  proceed  with  some  degree  of  or- 
der and  method,  you  will  in  the  end  find  yourself  but  lit- 
tle the  wiser  for  all  you  learn.  Let  us  therefore  go  on 
regularly,  and  make  ourselves  well  acquainted  with  the 
general  properties  of  bodies,  before  we  proceed  further. 

Emily,  To  return,  then,  to  attraction,  (which  appears 
to  me  by  far  the  most  interesting  of  them,  since  it  belongs 
equally  to  all  kinds  of  matter,)  it  must  be  mutual  between 
two  bodies ;  and  if  so,  when  a  stone  falls  to  the  earth,  the 
earth  should  rise  part  of  the  way  to  meet  the  stone  ? 

Mrs,  B.  Certainly  ;  but  you  must  recollect  that  the 
force  of  attraction  is  proportioned  to  the  quantity  of  mat- 
ter which  bodies  contain,  and  if  you  consider  the  differ- 
ence there  is  in  that  respect,  between  a  stone  and  the 
earth,  you  will  not  be  surprised  that  you  do  not  perceive 
the  earth  rise  to  meet  the  stone  ;  for  though  it  is  true  that  a 
mutual  attraction  takes  place  between  the  earth  and  the 
stone,  that  of  the  latter  is  so  very  small  in  comparison  to 
to  that  of  the  former,  as  to  render  its  effect  insensible. 

Emily,  But  since  attraction  is  proportioned  to  the 
quantity  of  matter  which  bodies  contain,  why  do  not  the 
hills  attract  the  houses  and  churches  towards  them  ? 

Caroline.  You  surprise  me,  Emily  ;  what  an  idea ! 
How  can  the  houses  and  churches  be  moved,  when  they 
are  so  firmly  fixed  in  the  ground  ? 

Mrs.  B.  Emily's  question  is  not  absurd,  and  your 
answer,  Caroline,  is  perfectly  just ;  but  can  you  tell  us 
why  the  houses  and  churches  are  so  firmly  fixed  in  the 
ground. 

Caroline.  I  am  afraid  I  have  ansv/ered  right  by  mere 
chance  ;  for  I  begin  to  suspect  that  bricklayers  and  car- 
penters could  give  but  little  stability  to  their  buildings, 
without  the  aid  of  attraction. 


73.  If  the  attraction  of  gravitation  is  mutual  between  bodies, 
why  do  we  not  see  the  earth  rise  part  way  to  meet  the  stone 
'^hich  ig  falling  towards  it  ? 


28  ON  THE  ATTRACTION  OP  GRAVITY. 

Mrs.  B.  It  is  certainly  the  cohesive  attraction  between 
the  bricks  and  the  mortar  which  enables  them  to  build 
walls,  and  these  are  so  strongly  attracted  hjr  the  earth,  as 
to  resist  every  other  impulse  ;  otherwise  they  would  ne- 
cessarily move  towards  the  hills  and  the  mountains  ;  but 
the  lesser  force  must  yield  to  the  greater.  There  are,  how- 
ever, some  circumstances  iQ  which  the  attraction  of  a  large 
body  has  sensibly  counteracted  that  of  the  earth.  If, 
whilst  standing  on  the  declivity  of  a  mountain,  you  hold  a 
plumb-line  in  your  hand,  the  weight  will  not  fall  perpen- 
dicular to  the  earth,  but  incline  a  little  towards  the  moun- 
tain ;  and  this  is  owing  to  the  lateral,  or  sideways  attrac- 
tion of  the  mountain,  interfering  with  the  perpendicular 
attraction  of  the  earth. 

Emily.  But  the  size  of  a  mountain  is  very  trifling 
compared  to  the  whole  earth  ? 

Mrs.  B.  Attraction,  you  must  recollect,  diminishes 
with  distance  ;  and  in  the  example  of  the  plumb-line,  the 
weight  suspended  is  considerably  nearer  to  the  mountain 
than  to  the  centre  of  the  earth  1 

Caroline.  Pray,  Mrs.  B.,  do  the  two  scales  of  a  ba- 
lance hang  parallel  to  each  other  1 

Mrs.  B.  You  mean,  I  suppose,  in  other  words,  to  in- 
quire whether  two  lines  which  are  perpendicular  to  the 
earthy  are  parallel  to  each  other  ?  I  believe  I  guess  the 
reason  of  your  question  ;  but  I  wish  you  would  endeavour 
to  answer  it  without  my  assistance. 

Caroline.  I  was  thinking  that  such  lines  must  both 
tend  by  gravity  to  the  same  point,  the  centre  of  the  earth  ; 
now  lines  tending  to  the  same  point  cannot  be  parallel,  as 
parallel  lines  are  always  at  an  equal  distance  from  each 
Other,  and  would  never  meet. 

Mrs.  B.  Very  well  explained  ;  you  see  now  the  use 
of  your  knowledge  of  parallel  lines  :  had  you  been  igno- 
rant of  their  properties,  you  could  not  have  drawn  such 
a  conclusion.  This  may  enable  you  to  form  an  idea  of 
the  great  advantage  to  be  derived  even  from  a  slight 
knowledge  of  geometry  in  the  study  of  natural  philoso- 
phy ;  and  if,  after  I  have  made  you  acquainted  with  the 
first  elements,  you  should  be  tempted  to  pursue  the  study, 

74.  And  why  are  not  houses  and  other  objects  at  the  side  of  a 
mountain  attracted  or  drawn  away  from  their  foundations  towards 
it  ? 75.  How  can  it  be  shown  that  mountains  possess  a  side- 
ways attraction  ? 76.     Would  two  lines  suspended  by  weio-hts 

be  parallel  to  each  other  ' 


ON  THE  ATTRACTION  OP  GRAVITY.  29 

I  would  advise  you  to  prepare  yourselves  by  acquiring 
some  knowledge  of  geometry.  This  science  would  teach 
you  that  lines  which  fall  perpendicular  to  the  surface  of  a 
sphere  cannot  be  parallel,  because  they  would  all  meet,  if 
prolonged  to  the  centre  of  the  sphere  ;  while  lines  that 
fall  pel-pendicular  to  a  plane  or  fiat  surface,  are  always 
parallel,  because,  if  prolonged,  they  v/ould  never  meet. 

Emily.  And  yet  a  pair  of  scales,  hanging  perpendicu- 
lar to  the  earth,  appear  parallel  ? 

3Irs.  B.  Because  the  sphere  is  so  large,  and  the  scales 
consequently  converge  so  little,  that  their  inclination  is 
not  perceptible  to  our  senses ;  if  we  could  construct  a 
pair  of  scales  whose  beam  would  extend  several  degrees, 
their  convergence  would  be  very  obvious  ;  but  as  this 
cannot  be  accomplished,  let  us  draw  a  small  figure  of  the 
earth,  and  then  we  may  make  a  pair  of  scales  of  the  pro- 
portion we  please,  (fig.  1.  plate  1.) 

Caroline.     This  figure  renders  it  very  clear  :  then  two 
bodies  cannot  fall  to  the  earth  in  parallel  lines  ? 
Mrs,  B.     Never. 

Caroline,  The  reason  that  a  heavy  body  falls  quicker 
than  a  light  one,  is,  I  suppose,  because  the  earth  attracts 
it  more  strongly  T 

3Irs.  B,  The  earth,  it  is  true,  attracts  a  heavy  body 
more  than  a  light  one  ;  but  that  would  not  make  the  one 
fall  quicker  than  the  other. 

Caroline.  Yet  since  it  is  attraction  that  occasions  the 
fall  of  bodies,  surely  the  more  a  body  is  attracted,  the 
more  rapidly  it  will  fall.  Besides,  experience  proves  it  to 
be  so.  Do  we  not  every  day  see  heavy  bodies  fall  quickly, 
and  light  bodies  slowly  ? 

Emily.  It  strikes  me,  as  it  does  Caroline,  that  as  at- 
traction is  proportioned  to  the  quantity  of  matter,  the 
earth  must  necessarily  attract  a  body  which  contains  a 
great  quantity  more  strongly,  and  therefore  bring  it  to  the 
ground  sooner  than  one  consisting  of  a  smaller  quantity. 
Mrs.  B.  You  must  consider,  that  if  heavy  bodies  are 
attracted  more  strongly  than  light  ones,  they  require 
more  attraction  to  make  them  fall.     Remember  that  bo- 

77.     Why  would  they  not  be  ? 78.     Why  is  not  their  con- 

Tergency  perceptible  I 79.     What  fij^ure  illustra,tes  the  con- 

vergency  of  two  lines  suspended  perpendicularly  to  the  surface  of 

the  earth  ? 80.     Do  heavy  and  light  bodies  fall  to  the  ground 

with  equal  rapidity  ? 

3* 


30  ON  THE  ATTRACTION  OF  GRAVITF. 

dies  have  no  natural  tendency  to  fall,  any  more  than  to 
rise,  or  to  move  laterally,  and  that  they  will  not  fall  un- 
less impelled  by  some  force  ;  now  this  force  must  be  pro- 
portioned to  the  quantity  of  matter  it  has  to  move  :  a 
body  consisting  of  1000  particles  of  matter,  for  instance, 
requires  ten  times  as  much  attraction  to  bring  it  to  the 
ground  in  the  same  space  of  time  as  a  body  consisting  of 
only  100  particles. 

Caroline.  I  do  not  understand  that ;  for  it  seems  to 
me  that  the  heavier  a  body  is,  the  more  easily  and  rea- 
dily it  falls. 

Emily,  I  think  I  now  comprehend  it ;  let  me  try  if  1 
can  explain  it  to  Caroline.  Suppose  that  I  draw  towards 
me  two  weighty  bodies,  the  one  of  lOOlbs.,  the  other  of 
lOOOlbs.,  must  I  not  exert  ten  times  as  much  strength  to 
draw  the  larger  one  to  me,  in  the  same  space  of  time  as 
is  required  for  the  smaller  one  ?  And  if  the  earth  draw  a 
body  of  lOOOlbs.,  weight  to  it  in  the  same  space  of  time 
that  it  draws  a  body  of  lOOlbs.,  does  it  not  follow  that  it 
attracts  the  body  of  lOOOlbs.  weight  with  ten  times  the 
force  that  it  does  that  of  lOOlbs.  1 

Caroline,  I  comprehend  your  reasoning  perfectly  ;  but 
if  it  were  so,  the  body  of  lOOOlbs.  weight,  and  that  of  lOOlbs. 
would  fall  with  the  same  rapidity  ;  and  the  consequence 
would  be,  that  all  bodies,  whether  light  or  heavy,  being  at 
an  equal  distance  from  the  ground,  would  fall  to  it  in  the 
same  space  of  time :  now  it  is  very  evident  that  this  con- 
elusion  is  absurd  ;  experience  every  instant  contradicts  it ; 
observe  how  much  sooner  this  book  reaches  the  floor  than 
this  sheet  of  paper,  when  I  let  them  drop  together. 

Emily,  That  is  an  objection  I  cannot  answer.  I  must 
refer  it  to  you,  Mrs.  B. 

Mrs.  B.  I  trust  that  we  shall  not  find  it  insurmount- 
able. It  is  true  that,  according  to  the  laws  of  attraction, 
all  bodies  at  an  equal  distance  from  the  earth,  should  fall 
to  it  in  the  same  space  of  time  ;  and  this  would  actually 
take  place  if  no  obstacle  intervened  to  impede  their  fall. 
But  bodies  fall  through  the  air,  and  it  is  the  resistance 
of  the  air  which  makes  bodies  of  different  density 
fall    with   different   degrees    of    velocity.     They    must 

81.  To  what  must  the  force  of  gravity  be  proportional  neces- 
sary in  causing  bodies  of  different  weights  to  fall  to  the  ground  ? 

82.     What  are  the  laws  of  attraction  in  regard  to  the  falling 

of  bodies  at  equal  distances  from  the  earth  ? 83.    But  why  then 

do  heavy  bodies  fall  quicker  than  light  ones  ? 


ON  THE  ATTRACTION  OF  GRAVITY.  31 

all  force  their  way  through  the  air,  but  dense  heavy 
bodies  overcome  this  obstacle  more  easily  than  rarer 
and  lighter  ones. 

The  resistance  which  the  air  opposes  to  the  fall  of  bo- 
dies is  proportioned  to  their  surface,  not  to  their  weight ; 
the  air  being  inert,  cannot  exert  a  greater  force  to  support 
the  weight  of  a  cannon-ball,  than  it  does  to  support  the 
weight  of  a  ball  (of  the  same  size)  made  of  leather  ;  but 
the  cannon-ball  will  overcome  this  resistance  more  easily, 
and  fall  to  the  ground,  consequently,  quicker  than  the 
leather  ball. 

Caroline,  This  is  very  clear,  and  solves  the  difficulty 
perfectly.  The  air  offers  the  same  resistance  to  a  bit  of 
lead  and  a  bit  of  feather  of  the  same  size  ;  yet  the  one 
seems  to  meet  with  no  obstruction  in  its  fall,  whilst  the 
other  is  evidently  resisted  and  supported  for  some  time  by 
the  air. 

Emily,  The  larger  the  surface  of  a  body,  then,  the 
more  air  it  covers,  and  the  greater  is  the  resistance  it 
meets  with  from  it. 

Mrs,  B.  Certainly  ;  observe  the  manner  in  which 
this  sheet  of  paper  falls  ;  it  floats  awhile  in  '&iQ  air,  and 
then  gently  descends  to  the  ground.  I  will  roll  the  same 
piece  of  paper  up  into  a  ball  :  it  offers  now  but  a  small 
surface  to  the  air,  and  encounters  therefore  but  little  re- 
sistance :  see  how  much  more  rapidly  it  fcJls. 

The  heaviest  bodies  may  be  made  to  fiodt  awhile  in  the  air, 
by  making  the  extent  of  their  surface  counterbaiance  their 
weight.  Here  is  some  gold,  which  is  the  most  dense  body 
we  are  acquainted  with,  but  it  has  been  beaten  into  a  very 
thin  leaf,  and  offers  so  great  an  extent  of  surface  in  propor- 
tion to  its  weight,  that  its  fall,  yoa  see,  is  still  more  retarded 
by  the  resistance  of  the  air  than  that  of  the  sheet  of  paper. 

Caroline,  That  is  very  curious ;  and  it  is,  I  suppose, 
upon  the  same  principle  that  iron  boats  may  be  made  to 
float  on  water  ? 

But,  Mrs.  B.,  if  the  air  is  a  real  body,  is  it  not  also 
subjected  to  the  laws  of  gravity  ? 

Mrs,  B,     Undoubtedly. 

Caroline,  Then  why  does  it  not,  like  all  other  bodies, 
fall  to  the  ground  ? 

84.     To  what  is  the  resistance,  that  the  air  opposes  to  falling 

bodies,  proportioned  ? 85.     How  can  heavy  bodies  be  made  to 

float  awhile  in  the  air  instead  of  falling  immediately  to  the  ground  ^ 
S6.     Does  the  air  gravitate  towards  the  earth  ^ 


32  eN  THE  ATTRACTION  OP  GRAVITY. 

Mrs.  B,  On  account  of  its  spring  or  elasticity.  The 
air  is  an  elastick  Jiuid ;  a  species  of  bodies,  the  peculiar 
property  of  which  is  to  resume,  after  compression,  their  ori- 
ginal dimensions  ;  and  you  must  consider  the  air  of  which 
the  atmosphere  is  composed  as  existing  in  a  state  of  com- 
pression, for  its  particles  bemg  drawn  towards  the  earth 
by  gravity,  are  brought  closer  togetJier  than  they  would 
otherwise  be,  but  the  spring  or  elasticity  of  the  air  by  which 
it  endeavours  to  resist  compression  gives  it  a  coTistant  ten- 
dency to  expand  itself,  so  as  to  resume  the  dimensions  it 
would  naturally  have,  if  not  under  the  iuPj.ience  of  gravity. 
The  air  may  therefore  be  said  r.onrtantly  to  struggle  with 
the  pov/er  of  gravity  without  being  able  to  overcom^e  it. 
Gravity  thus  confines  the  air  to  the  regions  of  our  globe, 
whilst  its  elasticity  prevents  it  from  fUlImg  like  other  bo- 
dies to  the  ground. 

Emily,  The  air  then  is,  I  suppose,  thicker,  or  I 
should  rather  say  more  dense,  near  the  surface  of  the  earth, 
than  in  the  higher  regions  of  the  atmosphere  ;  for  that  part 
of  the  air  which  is  nearer  the  surface  of  the  earth  must  be 
most  strongly  attracted. 

Mrs.  B.  The  diminution  of  the  force  of  gravity,  at  so 
small  a  distance  as  that  to  which  the  atmosphere  extends 
(compared  with  the  size  of  the  earth)  is  so  inconsiderable 
as  to  be  scarcely  sensible  ;  but  the  pressure  of  the  upper 
parts  of  the  atmosphere  on  those  beneath,  renders  the  air 
near  the  surface  of  the  earth  much  more  dense  than  the 
upper  regions. 

The  pressure  of  the  atmosphere  has  been  compared  to 
that  of  a  pile  of  fleeces  of  w  ool,  in  which  the  lower  fleeces 
are  pressed  together  by  the  w^eight  of  those  above  ;  these 
lie  light  and  loose,  in  proportion  as  they  approach  the  up- 
permost fleece,  which  receives  no  external  pressure,  and 
is  confined  merely  by  the  force  of  its  own  gravity. 

Caroliyic.  It  has  just  occurred  to  me  that  there  are  some 
bodies  w^hich  do  not  gravitate  towards  the  earth.  Smoke 
and  steam,  for  instance,  rise  instead  of  falling. 

87.     Why  then  does  it  not  fall  like  other  bodies  completely  to 

the  surface  of  the  earth  ? 88.     What  two  forces  continually 

operate  against  each  other  on  the  air  ? 89.     Is  the  air  of  the 

same  density  at  the  surface  of  the  earth  as  at  a  distance  from  it  ? 

90.    At  which  is  the  density  the  greatest  ? 91.    Why  is  the 

air  more  dense  at  the  surface  of  the  earth  than  at  a  distance  from 

it  ? 92.     To  what  has  the  pressure  of  the   atmosphere  been 

compared  f 


ON  THE  ATTRACTION  OF  GRAVITY.  33 

Mrs.  B,  It  is  still  gravity  which  produces  their  as- 
cent ;  at  least,  were  that  power  destroyed,  these  bodies 
would  not  rise. 

Caroline,  I  shall  be  out  of  conceit  with  gravity,  if  it 
is  so  inconsistent  in  its  operations. 

3Irs.  B.  There  is  no  difficulty  in  reconciling  this  ap- 
parent inconsistency  of  effect.  The  air  near  the  earth  is 
heavier  than  smoke,  steam,  or  other  vapours ;  it  conse- 
quently not  only  supports  these  light  bodies,  but  forces 
them  to  rise,  till  they  reach  a  part  of  the  atmosphere,  the 
weight  of  which  is  not  greater  than  their  own,  and  then 
they  remain  stationary.  Look  at  this  basin  of  water : 
why  does  the  piece  of  paper  which  I  throw  into  it  float 
on  the  surface  T 

Emily.  Because,  being  lighter  than  the  water,  it  is 
supported  by  it. 

Mrs.  B.  And  now  that  I  pour  more  water  into  the 
basin,  why  does  the  paper  rise  ? 

Emily.  The  water  being  heavier  than  the  paper,  gets 
beneath  it  and  obliges  it  to  rise. 

Mrs.  B.  In  a  similar  manner  are  smoke  and  vapour 
forced  upwards  by  the  air  ;  but  these  bodies  do  not,  like 
the  paper,  ascend  to  the  surface  of  the  fluid,  because,  as  we 
observed  before,  the  air  being  thinner  and  lighter  as  it  is 
more  distant  from  the  earth,vapours  rise  only  till  they  attain 
a  region  of  air  of  their  own  density.  Smoke , indeed, ascends 
but  a  very  little  way  ;  it  consists  of  minute  particles  of  fuel 
carried  up  by  a  current  of  heated  air  from  the  fire  below  : 
heat,  you  recollect,  expands  all  bodies  ;  it  consequently  ra- 
refies air,  and  renders  it  lighter  than  the  colder  air  of  the 
atmosphere  ;  the  heated  air  from  the  fire  carries  up  with  it 
vapour  and  small  particles  of  the  combustible  materials 
which  are  burning  in  the  fire.  When  this  current  of  hot  air 
is  cooled  by  mixing  with  that  of  the  atmosphere,  the  minute 
particles  of  coal  or  other  combustible  fall,  and  it  is  this 
which  produces  the  small  black  flakes  which  render  the  air 
and  every  thing  in  contact  with  it,  in  London,  so  dirty. 

Caroline.  You  must,  however,  allow  me  to  make  one 
more  objection  to  the  universal  gravity  of  bodies  ;  which 

93.     How  does  gfavity  operate  in  causing  smoke  and  steam  to 

rise  instead  of  falling  lo  the  earth  ? 94.     How  high  will  they 

rise  hefore  they  become  stationary  ? 95.  What  familiar  illus- 
tration is  given  of  the  principle  upon  which  smoke  and  vapour 
ascend  ^ 96.     Of  what  does  smoke  consist  ^ 


34  ON  THE  ATTRACTION  OF  GRAVITY. 

is  the  ascent  of  air  balloons,  the  materials  of  which  are 
undoubtedly  heavier  than  air  :  how,  therefore,  can  they 
be  supported  by  it  ? 

Mrs,  B.  I  admit  that  the  materials  of  which  balloons 
are  made  are  heavier  tlian  the  air  ;  but  the  air  with  which 
•they  are  filled  is  an  elastick  fluid,  of  a  different  nature  from 
the  atmospherick  air,  and  considerably  lighter  ;  so  that  on 
the  whole,  the  balloon  is  lighter  than  the  air  which  it  dis- 
places, and  consequently  will  rise,  on  the  same  principle  as 
smoke  and  vapour.  Now,  Emily,  let  me  hear  if  you  can 
explain  how  the  gravity  of  bodies  is  modified  by  the  effect 
of  the  air  ? 

Emihj.  The  air  forces  bodies  which  are  lighter  than 
itself  to  ascend ;  those  that  are  of  an  equal  weight  will 
remain  stationary  in  it ;  and  those  that  are  heavier  will 
descend  through  it ;  but  the  air  will  have  some  effect  on 
these  last ;  for  if  they  are  not  much  heavier,  they  will  with 
difficulty  overcome  the  resistance  they  meet  with  in  pass- 
ing through  it,  they  will  be  borne  up  by  it,  and  their  fall 
will  be  more  or  less  retarded. 

Mrs,  B.  Very  well.  Observe  how  slowly  this  light  feather 
falls  to  the  ground,  while  a  heavier  body,  like  this  marble, 
overcomes  the  resistance  which  the  air  makes  to  its  descent 
much  more  easily,  and  its  fall  is  proportionally  more  rapid. 
I  now  throw  a  pebble  into  this  tub  of  water ;  it  does  not  reach 
the  bottom  near  so  soon  as  if  there  were  no  water  in  the  tub» 
because  it  meets  with  resistance  from  the  water.  Suppose 
that  we  could  empty  the  tub, not  only  of  water, but  of  air  also, 
the  pebble  would  then  fall  quicker  still,  as  it  would  in  that 
case  meet  with  no  resistance  at  all  to  counteract  its  gravity. 

Thus  you  see  that  it  is  not  the  different  degrees  of 
gravity,  but  the  resistance  of  the  air,  which  prevents  bo- 
dies of  different  weight  from  fallmg  with  equal  velocities ; 
if  the  air  did  not  bear  up  the  feather,  it  would  reach  the 
ground  as  soon  as  the  marble. 

Caroline,  I  make  no  doubt  that  it  is  so  ;  and  yet  I  do 
not  feel  quite  satisfied.  I  wish  there  were  some  place 
void  of  air,  in  which  the  experiment  could  be  made. 

Mrs,  B.  If  that  proof  will  satisfy  your  doubts,  I  can 
give  it  you.  Here  is  a  machine  called  an  air  pump,  (fig.  2. 
pi.  I.)  by  means  of  which  the  air  may  be  expelled  from 

I.>7.     On  what  principle  does  a  balloon  rise,  since  it  is  made  of 

materials    heavier  than   the  air    through  which  it  rises  ? 98 

How  is  tliQ  t^ravity  of  bodies  modified  by  the  effect  of  the  air  P 
MO     What  is  the  uac  of  the  air  pump  ' 


ON  THE  ATTRACTION  OF  GRAVITY.  35 

any  close  vessel  which  is  placed  over  this  opening,  through 
which  the  air  is  pumped  out.  Glasses  of  various  shapes, 
usually  called  receivers,  are  employed  for  this  purpose. 
We  Siiall  naw  exhaust  the  air  from  this  tall  receiver  which 
is  placed  over  the  opening,  and  we  shall  find  that  bodies 
of  whatever  weight  or  size  vvithin  it,  will  fall  from  the  top 
to  the  bottom  in  the  same  space  of  time. 

Caroline,  Oh,  I  shall  be  delighted  with  this  experi- 
ment ;  what  a  curious  machine  !  how  can  you  put  the 
two  bodies  of  diiferent  weight  within  the  glass,  without 
admitting  the  air  ? 

Mrs,  B.  A  guinea  and  a  feather  are  already  placed 
there  for  the  purpose  of  the  experiment :  here  is,  you  see, 
a  contrivance  to  fasten  them  in  the  upper  part  of  the  glass  ; 
as  soon  as  the  air  is  pumped  out,  I  shall  turn  this  little 
screw,  by  which  means  the  brass  plates  which  support 
them  will  be  inclined,  and  the  two  bodies  will  fall. — Now 
I  believe  I  have  pretty  well  exhausted  the  air. 

Caroline,  Pray  let  me  turn  the  screw.  I  declare, 
they  both  reached  the  bottom  at  the  same  instant  !  Did 
you  see,  Emily,  the  feather  appeared  as  heavy  as  the 
guinea  ? 

Emily,  Exactly  ;  and  fell  just  as  quickly.  How  w^on- 
derful  this  is !  what  a  number  of  entertaining  experi- 
ments might  be  made  v/ith  this  machine  ! 

Mrs,  B,  No  doubt  there  are  a  great  many  ;  but  we 
shall  reserve  them  to  elucidate  the  subjects  to  which 
they  relate  ;  if  I  had  not  explained  to  you  why  the  guinea 
and  the  feather  fell  with  equal  velocity,  you  would  not 
have  been  so  well  pleased  with  the  experiment. 

Emily,  I  should  have  been  as  much  surprised,  but  not 
so  much  interested  ;  besides,  experiments  help  to  imprint 
on  the  memory  the  facts  they  are  intended  to  illustrate  ; 
it  will  be  better  therefore  for  us  to  restrain  our  curiosity, 
and  wait  for  other  experiments  in  their  proper  places. 

Caroline,  'Pray  by  what  means  is  the  air  exhausted  in 
this  receiver  ? 

Mi^s,  B,  You  must  learn  something  of  mechanicks  in 
order  to  understand  the  construction  of  a  pump.  At  our 
next  meeting,  therefore,  I  shall  endeavour  to  make  you 
acquainted  with  the  laws  of  motion,  as  an  introduction  to 
that  subject. 

100.  Can  a  feather  be  placed  in  a  situation  to  fall  as  quickly  as 
a  stone  ^ 101.     In  what  manner  can  it  be  done  ? 


36  ON  THE  LAWS  OF  MOTION. 

CONVERSATION  III. 

^  ON  THE  LAWS  OF  MOTION. 

On  Motion ;  Of  the  Inertia  of  Bodies ;  Of  Force  to 
produce  Motion ;  Direction  of  Motion  ;  Velocity^  Ah^ 
solute  and  Relative ;  Uniform  Motion ;  Retarded  Mo^ 
tion;  Accelerated  Motion;  Velocity  of  Falling  Bo- 
dies;  Momentum;  Action  and  Re-action  Equal; 
Elasticity  of  Bodies ;  Porosity  of  Bodies ;  Reflected 
Motion  ;  Angles  of  Incidence  and  Reflection, 

MRS.   B. 

The  science  of  mechanicks  is  founded  on  the  laws  of 
motion  ;  it  will,  therefore,  be  necessary  to  make  you  ac- 
quainted with  these  laws  before  we  examine  the  mecha- 
nical powers.  Tell  me,  Caroline,  what  do  you  understand 
by  the  word  motion  ? 

Caroline.  I  think  I  understand  it  perfectly,  though  I 
am  at  a  loss  to  describe  it.  Motion  is  the  act  of  moving 
about,  going  from  one  place  to  another  ;  it  is  the  contrary 
of  remaining  at  rest. 

Mrs,  B,  Very  well.  Motion  then  consists  in  a  change 
of  place  ;  a  body  is  in  motion  whenever  it  is  changing  its 
situation  with  regard  to  a  fixed  point. 

Now  since  we  have  observed  that  one  of  the  general 
properties  of  bodies  is  Inertia,  that  is,  an  entire  passiveness 
either  with  regard  to  motion  or  rest,  it  follows  that  a  body 
cannot  move  without  being  put  into  motion  ;  the  power 
which  puts  a  body  into  motion  is  called  force ;  thus,  the 
stroke  of  the  hammer  is  the  force  which  drives  the  nail ; 
the  pulling  of  the  horse  that  which  draws  the  carriage, 
&c.     Force  then  is  the  cause  which  produces  motion. 

Emily,  And  may  we  not  say  that  gravity  is  the  force 
which  occasions  the  fall  of  bodies  ? 

Mrs,  B,  Undoubtedly.  I  had  given  you  the  most  fa- 
miliar illustrations  in  order  to  render  the  explanation 
clear  ;  but  since  you  seek  for  more  scientifick  examples, 
you  may  say  that  cohesion  is  the  force  which  binds  the 
particles  of  bodies  together,  and  heat  that  which  drives 
them  asunder. 

102.     On  what  is  the  science  of  mechanicks  founded  ? 103. 

What  is  to  bo  understood  by  the  term  motion  ? 104.     What  is 

the  power  called  that  puts  a  body  in  motion  ? 


ON  THE  LAWS  OF  MOTION.  37 

The  motion  of  a  body  a^ted  upon  by  a  single  force  is 
always  in  a  straight  line,  in  the  direction  in  which  it  re- 
ceived the  impulse. 

Caroline,  That  is  very  natural ;  for  as  the  body  is  in- 
ert, and  can  move  only  because  it  is  impelled,  it  will  move 
only  in  the  direction  in  which  it  is  impelled.  The  degree 
of  quickness  with  wliich  it  moves,  must,  I  suppose,  also  de- 
pend upon  the  degree  of  force  with  which  it  is  impelled. 

Mrs*  B,  Yes  ;  the  rate  at  which  a  body  moves,  or  the 
shortness  of  the  time  which  it  takes  to  move  from  one 
place  to  another,  is  called  its  velocity  ;  and  it  is  one  of 
the  laws  of  motion  that  the  velocity  of  the  moving  body  is 
proportional  to  the  force  by  which  it  is  put  in  motion. 

We  must  distinguish  between  absolute  and  relative  ve- 
locity. 

The  velocity  of  a  body  is  called  absolute,  if  we  consider 
the  motion  of  the  body  in  space,  without  any  reference  to 
that  of  other  bodies.  When  for  instance  a  horse  goes  fifty 
miles  in  ten  hours,  his  velocity  is  five  miles  an  hour. 

The  velocity  of  a  body  is  termed  relative,  when  com- 
pared with  that  of  another  body  which  is  itself  in  motion. 
For  instance,  if  one  man  walks  at  the  rate  of  a  mile  an 
hour,  and  another  at  the  rate  of  two  miles  an  hour,  the 
relative  velocity  of  the  latter  is  double  that  of  the  former, 
but  the  absolute  velocity  of  the  one  is  one  mile,  and  that 
of  the  other  two  miles  an  hour. 

Emily.  Let  me  see  if  I  understand  it.  The  relative 
velocity  of  a  body  is  the  degree  of  rapidity  of  its  motion 
compared  with  that  of  another  body  ;  thus,  if  one  ship 
sail  three  times  as  far  as  another  ship  in  the  same  space 
of  time,  the  velocity  of  the  former  is  equal  to  three  times 
that  of  the  latter. 

Mrs.  B,  The  general  rule  may  be  expressed  thus  : 
the  velocity  of  a  body  is  measured  by  the  space  over 
which  it  moves,  divided  by  the  time  which  it  employs  in 
that  motion  :  thus  if  you  travel  one  hundred  miles  in 
twenty  hours,  what  is  your  velocity  in  each  hour  1 


105.     In  what  direction  is  the  motion  of  a  body  acted  on  by.  a 
single  force  ? 106.    What  is  meant  by  the  velocity  of  motion  .'* 

107.  To  what  is  the  velocity  of  a  moving  body  proportional  ? 

108.  What   is  called  absolute  velocity  ? 109.     When  is  the 

velocity  of  a  moving-  body  called  relative  ? 110.     What  would 

be  instances  of  relative  velocity  ^ 111.     What  is  the  general 

rule  for  calculating  the  velocity  of  a  moving  body  ? 

4 


38  ON  THE  LAWS  OF  MOTION. 

Emily,  I  must  divide  the  space,  which  is  one  hundred 
miles,  by  the  time,  which  is  twenty  hours,  and  the  answer 
will  be  '^\e  miles  an  hour.  Then,  Mrs.  B.,  may  we  not 
reverse  this  rule  and  say,  that  the  time  is  equal  to  the 
space  divided  by  the  velocity  ;  since  the  space  one  hun- 
dred miles,  divided  by  the  velocity  five  miles,  gives  twen- 
ty hours  for  the  time  ? 

Mrs.  B,  Certainly  ;  and  we  may  say  also  that  space 
is  equal  to  the  velocity  multiplied  by  the  time.  Can  you 
tell  me,  Caroline,  how  many  miles  you  will  have  travelled, 
if  your  velocity  is  three  miles  an  hour,  and  you  travel  six 
hours  ? 

Caroline.  Eighteen  miles  ;  for  the  product  of  3  mul- 
tiplied by  6,  is  18. 

Mrs.  B.  I  suppose  that  you  understand  what  is  meant 
by  the  terms  uniform,  accelerated,  and  retarded  motion. 

Emily.  I  conceive  uniform  motion  to  be  that  of  a  body 
whose  motion  is  regular,  and  at  an  equal  rate  throughout ; 
for  instance,  a  horse  that  goes  an  equal  number  of  miles 
every  hour.  But  the  hand  of  a  watch  is  a  much  better 
example,  as  its  motion  is  so  regular  as  to  indicate  the  time. 

Mrs.  B.  You  have  a  right  idea  of  uniform  motion  ; 
but  it  would  be  more  correctly  expressed  by  saying,  that 
the  motion  of  a  body  is  uniform  when  it  passes  over  equal 
spaces  in  equal  times.  Uniform  motion  is  produced  by 
a  force  having  acted  on  a  body  once,  and  having  ceased 
to  act ;  as  for  instance,  the  stroke  of  a  bat  on  a  cricket 
ball. 

Caroline.  But  the  motion  of  a  cricket  ball  is  not  uni- 
form ;  its  velocity  gradually  diminishes  till  it  falls  to  the 
ground. 

Mrs.  B.  Recollect  that  the  cricket  ball  is  inert,  and 
has  no  more  power  to  stop  than  to  put  itself  in  motion  ;  if 
it  falls,  therefore,  it  must  be  stopped  by  some  force  supe- 
riour  to  that  by  which  it  was  projected,  and  which  destroys 
its  motion. 

Caroline.  And  it  is  no  doubt  the  force  of  gravity  which 
counteracts  and  destroys  that  of  projection  ;  but  if  there 
were  no  such  power  as  gravity,  would  the  cricket  ball 
never  stop  ? 

Mrs.  B.  If  neither  gravity  nor  any  other  force,  such 
as  the  resistance  of  the  air,  opposed  its  motion,  the  cricket 

X12.     When  is  the  motion  of  a  body  termed  uniform  ? 113. 

How  is  uniform  motion  produced  ? 


ON  THE  LAWS  OF  MOTION.  39 

ball,  or  even  a  stone  thrown  by  the  hand,  would  proceed 
onwards  in  a  right  line,  and  with  a  uniform  velocity  for 
ever. 

Caroline,  You  astonish  me  !  I  thought  that  it  was  im- 
possible to  produce  perpetual  motion  ? 

Mrs.  B,  Perpetual  motion  cannot  be  produced  by  art, 
because  gravity  ultimately  destroys  all  motion  that  hu- 
man powers  can  produce. 

Emily.  But  independently  of  the  force  of  gravity,  I 
cannot  conceive  that  the  little  motion  I  am  capable  of 
giving  to  a  stone  would  put  it  in  motion  for  ever. 

3Irs,  B.  The  quantity  of  motion  you  communicate  to 
the  stone  would  not  influence  its  duration :  if  you  threw 
it  with  little  force  it  would  move  slowly  ;  for  its  velocity, 
you  must  remember,  will  be  proportional  to  the  force  with 
which  it  is  projected  ;  but  if  there  is  nothing  to  obstruct  its 
passage,  it  will  continue  to  move  with  the  same  velocity, 
and  in  the  same  direction  as  when  you  first  projected  it. 

Caroline,  This  appears  to  me  quite  incomprehensible ; 
we  do  not  meet  with  a  single  instance  of  it  in  nature. 

Mrs,  B,  I  beg  your  pardon.  When  you  come  to 
study  the  motion  of  the  celestial  bodies,  you  will  find  that 
nature  abounds  with  examples  of  perpetual  motion  ;  and 
that  it  conduces  as  much  to  the  harmony  of  the  system  of 
the  universe  as  the  prevalence  of  it  would  to  the  destruc- 
tion of  all  comfort  on  our  globe.  The  wisdom  of  Provi- 
dence has  therefore  ordained  insurmountable  obstacles  to 
perpetual  motion  here  below  ;  and  though  these  obstacles 
often  compel  us  to  contend  with  great  difficulties,  yet 
there  results  from  it  that  order,  regularity,  and  repose,  so 
essential  to  the  preservation  of  all  the  various  beings  of 
which  this  world  is  composed. 

Now  can  you  tell  me  what  is  retarded  motion  ? 

Caroline.  Retarded  motion  is  that  of  a  body  which 
moves  every  moment  slower  and  slower  :  thus  when  I 
am  tired  with  walking  fast,  I  slacken  my  pace  ;  or  when 
a  stone  is  thrown  upwards,  its  velocity  is  gradually  di- 
minished by  the  power  of  gravity. 

Mrs,  B.  Retarded  motion  is  produced  by  some  force 
acting  upon  the  body  in  a  direction  opposite  to  that  which 
first  put  it  in  motion  :  you  who  are  an  animated  being, 
endowed  with  power  and  will,  may  slacken  your  pace,  or 

114.  What  is  the  reason  that  perpetual  motion  cannot  be  pro- 
duced ? -U5.  What  is  retarded  motion  ? 116.  How  is  re- 
tarded motion  produced  ? 


40  ON  THE  LAWS  OP  MOTION* 

stop  to  rest  when  you  are  tired  ;  but  inert  matter  is  inca- 
pable of  any  feeling  of  fatigue,  can  never  slacken  its  pace 
and  never  stop,  unless  retarded  or  arrested  in  its  course 
.  by  some  opposing  force ;  and  as  it  is  the  Islws  of  inert 
bodies  which  mechanicks  treat  of,  I  prefer  your  illustra- 
tion of  the  stone  retarded  in  its  ascent.  Now,  Emily,  it 
is  your  turn  ;  what  is  accelei' cited  motion  ? 

Emily,  Accelerated  motion,  I  suppose,  takes  place 
when  the  velocity  of  a  body  is  increased  ;  if  you  had  not 
objected  to  our  giving  such  active  bodies  as  ourselves  as 
examples,  I  should  say  that  my  motion  is  accelerated  if  I 
ciiange  my  pace  from  walking  to  running.  I  cannot  think 
of  any  instance  of  accelerated  motion  in  inanimate  bodies  ; 
all  motion  of  inert  matter  seems  to  be  retarded  by  gravity. 

Mrs.  B.  Not  in  all  cases  ;  for  the  power  of  gravitation 
sometimes  produces  accelerated  motion ;  for  instance,  a 
stone  falling  from  a  height  moves  with  a  regularly  acce- 
lerated motion. 

Emily.  True  ;  because  the  nearer  it  approaches  the 
earth,  the  more  it  is  attracted  by  it. 

Mrs.  B.  You  have  mistaken  the  cause  of  its  accele- 
ration of  motion  ;  for  though  it  is  true  that  the  force  of 
gravity  increases  as  a  body  approaches  the  earth,  the  dif- 
ference is  so  trifling  at  any  small  distance  from  its  surface 
as  not  to  be  perceptible. 

Accelerated  motion  is  produced  when  the  force  which 
put  a  body  in  motion  continues  to  act  upon  it  during^ 
its  motion,  so  that  its  motion  is  continuaJly  increased. 
When  a  stone  falls  from  a  height,  the  impulse  which  it  re- 
ceives from  gravity  during  the  first  instant  of  its  fall,  would 
be  sufficient  to  bring  it  to  the  ground  with  a  uniform  ve- 
locity :  for,  as  we  have  observed,  a  body  having  been  once 
acted  upon  by  a  force,  will  continue  to  move  with  a  uni- 
form velocity  ;  but  the  stone  is  not  acted  upon  by  gravity 
merely  at  the  first  instant  of  its  fall — this  power  continues 
to  impel  it  during  the  whole  of  its  descent,  and  it  is  this 
continued  impulse  which  accelerates  its  motion. 

Emily.     1  do  not  quite  undertand  that. 

Mrs.  B.  Let  us  suppose  that  the  instant  after  you 
have  let  fall  a  stone  from  a  high  tower,  the  force  of  gra- 
vity were  annihilated,  the  body  would  nevertheless  con- 

117.  What  is  accelerated  motion? IJ8.  What  is  an  in- 
stance of  accelerated  motion  ? 119.  How  does  gravity  accele- 
rate the  motion  of  falling  bodies  ? 


ON  THE  LAWS  OF  MOTION.  41 

tinue  to  move  downwards,  for  it  would  have  received  a 
first  impulse  from  gravity,  and  a  body  once  put  in  motion 
will  not  stop  unless  it  meets  with  some  obstacle  to  impede 
its  course  ;  in  this  case  its  velocity  would  be  uniform,  for 
though  there  would  be  no  obstacle  to  obstruct  its  descent, 
there  would  be  no  force  to  accelerate  it. 

Emily,     That  is  very  clear. 

Mrs.  B.  Then  you  have  only  to  add  the  power  of 
gravity  constantly  acting  on  the  stone  during  its  descent, 
and  it  will  not  be  difficult  to  understand  that  its  motion 
will  become  accelerated,  since  the  gravity  which  acts  on 
the  stone  during  the  first  instant  of  its  descent,  will  con- 
tinue in  force  every  instant  till  it  reaches  the  ground. 
Let  us  suppose  that  the  impulse  given  by  gravity  to  the 
stone  during  the  first  instant  of  its  descent  be  equal  to  one, 
the  next  instant  we  shall  find  that  an  additional  impulse 
gives  the  stone  an  additional  velocity  equal  to  one,  so 
that  the  accumulated  velocity  is  now  equal  to  two  ;  the 
following  instant  another  impulse  increases  the  velocity  to 
three,  and  so  on  till  the  stone  reaches  the  ground. 

Caroline,  Now  I  understand  it ;  the  effects  of  preced 
ing  impulses  must  be  added  to  the  subsequent  velocities. 

Mrs.  B.  Yes  ;  it  has  been  ascertained  both  by  expe- 
riment and  calculations,  which  it  would  be  too  difficult  for 
us  to  enter  into,  that  heavy  bodies  descending  from  a  height 
by  the  force  of  gravity,  fall  sixteen  feet  the  first  second  of 
time,  three  times  that  distance  in  the  next,  five  times  in  the 
third  second,  seven  times  in  the  fourth,  and  so  on,  regu- 
larly increasing  their  velocities  according  to  the  number 
of  seconds  during  which  the  body  has  been  falling. 

Emily.  If  you  throw  a  stone  perpendicularly  upwards, 
is  it  not  the  same  length  of  time  ascending  that  it  is  de- 
scending ? 

Mrs.  B.  Exactly  ;  in  ascending,  the  velocity  is  di- 
minished by  the  force  of  gravity  ;  in  descending,  it  is  ac- 
celerated by  it. 

Caroline.  I  should  then  have  imagined  that  it  would 
have  fallen  quicker  than  it  rose  1 

Mrs.  B.  You  must  recollect  that  the  force  with  which 
it  is  projected  must  be  taken  into  the  account ;  and  that 

120.     What  distance  will  a  heavy  body,  suspended  in  the  air, 
fall  the  first  second  of  time  ?     What  distance  the  second  ?     What 
the  third  ? 121.     How  does  the  time  of  an  ascending  body  al- 
ways compare  with  the  time  of  its  descent  ? 
4* 


42  ON  THE  LAW&  OF  MOTION. 

this  force  is  overcome  and  destroyed  by  gravity  before  the 
body  falls. 

Caroline.  But  the  force  of  projection  given  to  a  stone 
in  throwing  it  upwards,  cannot  always  be  equal  to  the 
force  of  gravity  in  bringing  it  down  again,  for  the  force 
of  gravity  is  always  the  same,  whilst  the  degree  of  im- 
pulse  given  to  the  stone  is  optional  ;  I  may  throw  it  up 
gently  or  w  ith  violence. 

Mrs,  B.  If  you  throw  it  gently,  it  will  not  rise  high ; 
perhaps  only  sixteen  feet,  in  which  case  it  will  fall  in  one 
second  of  time.  Now  it  is  proved  by  experiment,  that  an 
impulse  requisite  to  project  a  body  sixteen  feet  upwards, 
will  make  it  ascend  that  height  in  one  second  ;  here  then 
the  times  of  the  ascent  and  descent  are  equal.  ^  But  sup- 
posing it  be  required  to  throw  a  stone  twice  that  height, 
the  force  must  be  proportionally  greater. 

You  see  then,  that  the  impulse  of  projection  in  throw- 
ing a  body  upwards,  is  always  equal  to  the  action  of  the 
force  of  gravity  during  its  descent  ;  and  that  it  is  the 
greater  or  less  distance  to  which  the  body  rises,  that 
makes  these  two  forces  balance  each  other. 

I  must  now  explain  to  you  what  is  meant  by  the  mo- 
mentiim  of  bodies.  It  is  the  force,  or  power,  with  which 
a  body  in  motion,  strikes  against  another  body.  The 
momentum  of  a  body  is  composed  of  its  quantity  of 
matter^  multiplied  by  its  quantity  of  motion ;  in  other 
words  its  weight  and  its  velocity. 

Caroline,  The  quicker  a  body  moves,  the  greater,  no 
doubt,  must  be  the  force  with  which  it  would  strike  against 
another  body. 

Kmihj.  Therefore  a  small  body  may  have  a  greater  mo- 
mentum than  a  large  one,  provided  its  velocity  be  sufficient- 
ly greater ;  for  instance,  the  momentum  of  an  arrow  shot 
from  a  bow  must  be  greater  than  a  stone  thrown  by  the  hand. 

Caroline.  We  know  also  by  experience,  that  the 
heavier  a  body  is,  the  greater  is  its  force ;  it  is  not  there- 
fore difficult  to  understand,  that  the  whole  power  or  mo- 
mentum of  a  ])ody  must  be  composed  of  these  two  pro- 
perties ;    but    I  do    not  understand,    why    they    should 

122.     To  what  is  the  impulse  of  projection,  in  throwing  a  body 

upwards,  equal? 123-     What  is  the   momentum  of  a  body  .^ 

124.     Of  what  is  the  momentum  of  a  body  composed  ^ 

125.  In  what  way  can  a  smaller  body  have  a  greater  moraentunt 
than  a  larger  body  ? 


ON  THE  LAWS  OP  MOTION.  43 

be  multiplied,  the  one  by  the  other  ;  I  should  have  sup- 
posed that  the  quantity  of  matter  should  have  been  added 
to  the  quantity  of  motion  1 

Mrs,  B.  It  is  found  by  experiment,  that  if  the  weight 
of  a  body  is  represented  by  the  number  3,  and  its  velocity 
also  by  3,  its  momentum  will  be  represented  by  9 ;  not  6, 
as  would  be  the  case,  were  these  figures  added,  instead  of 
being  multiplied  together.  I  recommend  it  to  you  to  be 
careful  to  remember  the  definition  of  the  momentum  of 
bodies,  as  it  is  one  of  the  most  important  points  in  mecha- 
nicks ;  you  will  find,  that  it  is  from  opposing  motion  to 
matter,  that  machines  derive  their  powers.* 

The  re-action  of  bodies  is  the  next  law  of  motion  which 
I  must  explain  to  you.  When  a  body  in  motion  strikes 
against  another  body,  it  meets  with  resistance  from  it ; 
the  resistance  of  the  body  at  rest  will  be  equal  to  the 
blow  struck  by  the  body  in  motion ;  or  to  express  myself 
in  philosophical  language,  action  and  re-action  will  be 
equal,  and  in  opposite  directions. 

Caroline,  Do  you  mean  to  say,  that  the  action  of  the 
body  which  strikes,  is  returned  with  equal  force  by  the 
body  which  receives  the  blow  1 

Mrs.  B.     Exactly. 

Caroline,  But  if  a  man  strikes  another  on  the  face  with 
his  fist,  he  surely  does  not  receive  as  much  pain  by  the 
re-action  as  he  inflicts  by  the  blow  1 

Mrs,  B,  No ;  but  this  is  simply  owing  to  the  knuckles 
having  much  less  feeling  than  the  face. 

Here  are  two  ivory  balls  suspended  by  threads,  (plate 
1.  fig.  3.)  draw  one  of  them.  A,  a  little  on  one  side, — now 
let  it  go  ; — it  strikes  you  see  against  the  other  ball  B,  and 
drives  it  off,  to  a  distance  equal  to  that  through  which  the 

*  In  comparing  together  the  momenta  of  different  bodies,  we 
must  be  attentive  to  measure  their  weights  and  velocities,  by  tha 
same  denomination  of  weights  and  of  spaces,  otherwise  the  results 
would  not  agree.  Thus  if  we  estimate  the  weight  of  one  body  in 
ounces,  v/e  must  estimate  the  weight  of  the  rest  also  in  ounces, 
and  not  in  pounds  ;  and  in  computing  the  velocities,  in  like  man- 
ner, we  should  adhere  to  the  same  standard  of  measure,  both  of 
space  and  of  time  ;  as  for  instance,  the  number  of  feet  in  one  se- 
cond, or  of  miles  in  one  hour. 

126.     If  the  weight  of  a  body  be  respresented  by  3,  and  its  ve- 

locity   by  3,  what    will   be    it-   momentum  ? 127.     WJiat    is 

meant  by  the  term  re-action,  in  mechanicks  ? 128.     To  what  is 

re-action  equal  ? 129.     What  does  figure  3,  Plate  T.  illustrate  ? 


44  ON  THE  LAWS  OF  MOTION. 

first  ball  fell ;  but  the  motion  of  A  is  stopped,  because 
when  it  struck  B,  it  received  in  return  a  blow  equal  to 
that  it  gave,  and  its  motion  was  consequently  destroyed. 

Emily.  I  should  have  supposed  that  the  motion  of  the 
ball  A  was  destroyed,  because  it  had  communicated  all 
its  motion  to  B. 

Mrs,  B.  It  is  perfectly  true,  that  when  one  body 
strikes  against  another,  the  quantity  of  motion  communi- 
cated to  the  second  body,  is  lost  by  the  first ;  but  this  loss 
proceeds  from  the  action  of  the  body  which  is  struck. 

Here  are  six  ivory  balls  hanging  in  a  row,  (fig.  4.)  draw 
the  first  out  of  the  perpendicular,  and  let  it  fall  against 
the  second.  None  of  the  balls  appear  to  move,  you  see, 
except  the  last,  which  flies  off  as  far  as  the  first  ball  fell  ; 
can  you  explain  this  1 

Caroline,  I  believe  so.  When  the  first  ball  struck  the 
second,  it  received  a  blow  in  return,  which  destroyed  its 
motion  ;  the  second  ball,  though  it  did  not  appear  to  move, 
must  have  struck  against  the  third;  the  re-action  of  which 
set  it  at  rest ;  the  action  of  the  third  ball  must  have  been 
destroyed  by  the  re-action  of  the  fourth,  and  so  on  till  mo- 
tion was  communicated  to  the  last  ball,  which,  not  being 
re-acted  upon,  flies  off. 

3Irs,  B,  Very  well  explained.  Observe,  that  it  is 
only  when  bodies  are  elastick,  as  these  ivory  balls  are,  that 
the  stroke  returned  is  equal  to  the  stroke  given.  I  will 
show  you  the  difference  with  these  two  balls  of  clay,  (fig. 
5.)  which  are  not  elastick  ;  when  you  raise  one  of  these, 

D,  out  of  the  perpendicular,  and  let  it  fall  against  the  other, 

E,  the  re-action  of  the  latter,  on  account  of  its  not  being 
elastick,  is  not  sufficient  to  destroy  the  motion  of  the  for- 
mer ;  only  part  of  the  motion  of  D  will  be  communicated  to 
E,  and  the  two  balls  will  move  on  together  to  d  and  e  which 
is  not  so  great  a  distance  as  that  through  which  D  fell. 

Observe  how  useful  re-action  is  in  nature.  Birds  in  fly- 
ing strike  the  air  with  their  wings,  and  it  is  the  re- action  of 
the  air  which  enables  them  to  rise,  or  advance  forwards ; 
re-action  being  always  in  a  contrary  direction  to  action. 

130.  How  would  you  explain  the  operation  of  action  and  re- 
action, as  illustrated  by  the  six  ivory  balls  in  Figure  4,  Plate  I.  ? 

131 .     Is  the  re-action  of  all  bodies  equal  to  the  action  when 

a  blow   is  given  ? 132.     In   what   ones  is  it  equal .'' 133. 

What  is  the  object  of  figure  5,  Plate  I.  .'' 134.     How  does  this 

figure  show  that  the  re-action  of  non-elastick  bodies  is  not  equal 

to  the  action  ? 135.     On  what  mechanical  principle  is  it  that 

birds  arc  able  to  %. 


ON  THE  LAWS  OP  MOTION.  45 

Caroline.  I  thought  that  birds  might  be  lighter  than 
the  air,  when  their  wings  were  expanded,  and  by  that 
means  enabled  to  fly. 

Mrs,  B.  When  their  wings  are  spread,  they  are  bet- 
ter supported  by  the  air,  as  they  cover  a  greater  extent  of 
surface  ;  but  they  are  still  much  too  heavy  to  remain  in  that 
situation,  without  continually  flapping  their  wings,  as  you 
may  have  noticed,  when  birds  hover  over  their  nests :  the 
force  with  which  their  wings  strike  against  the  air  must 
equal  the  weight  of  their  bodies,  in  order  that  the  re-action 
of  the  air  may  be  able  to  support  that  weight ;  the  bird  will 
then  remain  stationary.  If  the  stroke  of  the  wings  be 
greater  than  is  required  merely  to  support  the  bird,  the 
re-action  of  the  air  will  make  it  rise  ;  if  it  be  less,  it  will 
gently  descend  ;  and  you  may  have  observed  the  lark, 
sometimes  remaining  with  its  wings  extended,  but  mo- 
tionless :  in  this  state  it  drops  rapidly  into  its  nest. 

Caroline.  What  a  beautiful  effect  this  is  of  the  law  of 
re-action  !  But  if  flying  is  merely  a  mechanical  operation, 
Mrs.  B.,  why  should  we  not  construct  wings,  adapted  to 
the  size  of  our  bodies,  fasten  them  to  our  shoulders,  move 
them  with  our  arms,  and  soar  into  the  air. 

Mrs.  B.  Such  an  experiment  has  been  repeatedly  at- 
tempted, but  never  with  success  ;  and  it  is  now  considered 
as  totally  impracticable.  The  muscular  power  of  birds  is 
greater  in  proportion  to  their  weight  than  that  of  man  ; 
were  we  therefore  furnished  with  wings  sufficiently  large  to 
enable  us  to  fly,  we  should  not  have  strength  to  put  them  in 
motion.  In  swimming,  a  similar  action  is  produced  on  the 
water,  as  that  on  the  air  in  flying  ;  and  also  in  rowing  ;  you 
strike  the  water  with  the  oars,  in  a  direction  opposite  to  that 
in  which  the  boat  is  required  to  move  :  and  it  is  the  re-ac- 
tion of  the  water  on  the  oars  which  drives  the  boat  along. 

Emily.  You  said,  that  it  was  in  elastick  bodies  only, 
that  re-action  was  equal  to  action ;  pray  what  bodies  are 
elastick  besides  the   air. 

Mrs.  B.  In  speaking  of  the  air,  I  think  we  defined 
elasticity  to  be  a  property,  by  means  of  which,  bodies  that 
are  compressed  returned  to  their  former  state.     If  I  bend 

136,  How  must  a  bird  strike  the  air  with  its  wings  so  as  to  re- 
main stationary  ? — So  as  to  rise  ? — So   as  to    descend  ? 137. 

If  flying  is  only  the  effect  of  re-action,  why  could  not  a  man  bo  fur- 
nished with  wings  so  as  to  fly  ? 138.  How  is  swimming  effect- 
ed ? 139.     On  what  principle  is  a  boat  moved  upon  the  water  f 

-140.     What  is  to  be  understood  by  the  elasticity  of  a  body  - 


46  ON  THE  LAWS  OF  MOTION. 

this  cane,  as  soon  as  I  leave  it  at  liberty  it  recovers  its 
former  position  ;  if  I  press  my  finger  upon  your  arm,  as 
soon  as  I  remove  it,  the  flesh,  by  virtue  of  its  elasticity,  rises 
and  destroys  the  impression  I  made.  Of  all  bodies,  the 
air  is  the  most  eminent  for  this  property,  and  it  has  thence 
obtained  the  name  of  elastick  fluid.  Hard  bodies  are  in 
the  next  degree  elastick :  if  two  ivory,  or  metallic  balls 
are  struck  together,  the  parts  at  which  they  touch  will  be 
flattened  :  but  their  elasticity  will  make  them  instanta- 
neously resume  their  former  shape. 

Caroline.  But  when  two  ivory  balls  strike  against  each 
other,  as  they  constantly  do  on  a  billiard  table,  no  mark 
or  impression  is  made  by  the  stroke. 

Mrs,  B,  I  beg  your  pardon  ;  but  you  cannot  perceive 
any  mark,  because  their  elasticity  instantly  destroys  all 
trace  of  it. 

Soft  bodies,  which  easily  retain  impression,  such  as  clay, 
wax,  tallow,  butter,  &.c.  have  very  little  elasticity  ;  but  of 
all  descriptions  of  bodies  liquids  are  the  least  elastick. 

Emily,  If  sealing-wax  were  elastick,  instead  of  retain- 
ing the  impression  of  a  seal,  it  would  resume  a  smooth 
surface  as  soon  as  the  weight  of  the  seal  was  removed. 
But  pray  what  is  it  that  produces  the  elasticity  of  bodies  1 

Mrs.  B.  There  is  great  diversity  of  opinion  upon 
that  point,  and  I  cannot  pretend  to  decide  which  ap- 
proaches nearest  to  the  truth.  Elasticity  implies  suscep- 
tibility of  compression,  and  the  susceptibility  of  compres- 
sion depends  upon  the  porosity  of  bodies ;  for  were  there 
no  pores  or  spaces  between  the  particles  of  matter  of  which 
a  body  is  composed,  it  could  not  be  compressed. 

Caroline.  That  is  to  say,  that  if  the  particles  of  bodies 
were  as  close  together  as  possible,  they  could  not  b« 
squeezed  closer. 

Emily.  Bodies  then,  whose  particles  are  most  distant 
from  each  other,  must  be  most  susceptible  of  compression, 
and  consequently  most  elastick ;  and  this  you  say  is  the 
case  with  air,  which  is  perhaps  the  least  dense  of  all  bodies  ? 

Mrs.  B.  You  will  not  in  general  find  this  rule  hold 
good,  for  liquids  have  scarcely  any  elasticity,  whilst  hard 
bodies  are  eminent  for  this  property,  though  the  latter  are 
certainly  of  much  greater  density  than  the  former ;  elas- 

141.  What  bodies  are  most  distinguished  for  elasticity?—— 
142.  What  bodies  are  not  elastick.? 143.  On  what  is  elasti- 
city supposed  to  depend  r 


ON  THE  LAWS  OF  MOTION.  47 

ticity  implies,  therefore,  not  only  a  susceptibility  of  com- 
pression, but  depends  upon  the  power  of  resuming  its  for- 
mer state  after  compression. 

Caroline.  But  surely  there  can  be  no  pores  in  ivory 
and  metals,  Mrs.  B. ;  how  then  can  they  be  susceptible 
of  compression  ? 

Mrs.  B,  The  pores  of  such  bodies  are  invisible  to  the 
naked  eye,  but  you  must  not  thence  conclude  that  they 
have  none ;  it  is,  on  the  contrary,  well  asceitained  that 
gold,  one  of  the  most  dense  of  all  bodies,  is  extremely  po- 
rous, and  that  these  pores  are  sufficiently  large  to  admit 
water  when  strongly  compressed  to  pass  through  them. 

This  was  shown  by  a  celebrated  experiment  made 
many  years  ago  at  Florence. 

Emily.  If  water  can  pass  through  gold,  there  must 
certainly  be  pores  or  interstices  which  afford  it  a  passage  ; 
and  if  gold  is  so  porous,  what  must  other  bodies  be  which 
are  so  much  less  dense  than  gold ! 

3Trs.  B.  The  chief  difference  in  this  respect  is,  I  be- 
lieve, that  the  pores  in  some  bodies  are  larger  than  in 
others  ;  in  cork,  sponge,  and  bread,  they  form  considerable 
cavities ;  in  wood  and  stone,  when  not  polished,  they  are 
generally  perceptible  to  the  naked  eye  ;  whilst  in  ivory,  me- 
tals, and  all  varnished  and  polished  bodies,  they  cannot  be 
discerned.  To  give  you  an  idea  of  the  extreme  porosity  of 
bodies,  Sir  Isaac  Newton  conjectured  that  if  the  ear.th  were 
so  compressed  as  to  be  absolutely  without  pores,  its  dimen- 
sions might  possibly  not  be  more  than  a  cubic  inch. 

Caroline.  What  an  idea  \  Were  we  not  indebted  to 
Sir  Isaac  Newton  for  the  theory  of  attraction,  I  should  be 
tempted  to  laugh  at  him  for  such  a  supposition.  What 
insignificant  little  creatures  we  should  be  ! 

Mrs.  B.  If  our  consequence  arose  from  the  size  of 
our  bodies,  we  should  indeed  be  but  pigmies  ;  but  remem- 
ber that  the  mind  of  Newton  was  not  circumscribed  by 
the  dimensions  of  its  envelope. 

Emily.  It  is,  however,  fortunate  that  heat  keeps  the 
pores  of  matter  open  and  distended,  and  prevents  the  at- 
traction of  cohesion  from  squeezing  us  into  a  nut-shell. 

Mrs.  B.  Let  us  now  return  to  the  subject  of  re-action, 
on  which  we   have  some  further  observations  to  make. 

144.  Is  it  supposed  that  ivory  balls,  metals,  and  other  hard  sub- 
stances are  porous  ? 145.     How  has  it  been  proved  that  gold 

is  porous  ? 14(5.     What  conjecture  did  Sir  Isaac  Newton  form 

concerning  the  porosity  of  the  earth  .'* 


48**  ON  THE  LAWS  OF  MOTION. 

It  is  re-action,  being  contrary  to  action,  which  produces 
reflected  motion.  If*  you  throw  a  ball  against  the  wall,  it 
rebounds  ;  this  return  of  the  ball  is  owing  to  the  re-action 
of  the  wall  against  which  it  struck,  and  is  called  reflected 
motion, 

Emily,  And  I  now  understand  why  balls  filled  with 
air  rebound  better  than  those  stuffed  with  bran  and  wool, 
air  being  most  susceptible  of  compression  and  most  elas- 
tick,  the  re-action  is  more  complete. 

Caroline.  I  have  observed  that  when  I  throw  a  ball 
straight  against  the  wall,  it  returns  straight  to  my  hand  ; 
but  if  I  throw  it  obliquely  upwards,  it  rebounds  still  higher, 
and  I  catch  when  it  falls. 

Mrs,  B,  You  should  not  say  straight,  but  perpendi- 
cularly against  the  wall ;  for  straight  is  a  general  term  for 
lines  in  all  directions  which  are  neither  curved  nor  bent, 
and  is  therefore  equally  applicable  to  oblique  or  perpendi- 
cular lines. 

Caroline,  I  thought  that  perpendicularly  meant  either 
directly  upwards  or  downwards. 

Mrs,  B,  In  those  directions  lines  are  perpendicular 
to  the  earth.  A  perpendicular  line  has  always  a  reference 
to  something  towards  which  it  is  perpendicular ;  that  is  to 
say,  that  it  inclines  neither  to  the  one  side  nor  the  other,  but 
makes  an  equal  angle  on  every  side.  Do  you  understand 
what  an  angle  is  ? 

Caroline,  Yes,  I  believe  so  :  it  is  two  lines  meeting  in 
a  point. 

Mrs,  B,  Well  then,  let  the  line  A  B  (plate  II,  fig.  1 ,)  re- 
present the  floor  of  the  room,  and  the  line  C  D  that  in  which 
you  throw  a  ball  against  it :  the  line  C  D,  you  v/ill  observe, 
forms  two  angles  with  the  line  A  B,  and  those  two  angles 
are  equal. 

Emihj,  How  can  the  angles  be  equal,  while  the  lines 
which  compose  them  are  of  unequal  length  ? 

Mrs,  B,  An  angle  is  not  measured  by  the  length  of 
the  lines,  but  by  their  opening. 

Emihj,  Yet  the  longer  the  lines  are,  the  greater  is  the 
opening  between  them. 

Mrs,  B,  Take  a  pair  of  compasses  and  draw  a  circle 
over  these  angles,  making  the  angular  point  the  centre. 

147.     What  is  reflected  motion  ? 148.     What  produces  it  .^ 

. 149.    AVhat  is  meant  by  a  perpendicular  line  ^ 150.    What 

is  an  angle  1 151.     What  does  Fig.  I,  plate  II.  illustrate  .?-- — 

15^.     By  what  is  an  angle  measured  ? 


ON  THE  LAWS  OF  MOTION.  49 

Emily.     To  what  extent  must  I  open  the  compasses  1 

Mrs.  B.  You  may  draw  the  circle  what  size  you 
please,  provided  that  it  cuts  the  lines  of  the  angles  we 
are  to  measure.  All  circles,  of  whatever  dimensions,  are 
supposed  to  be  divided  into  360  equal  parts,  called  de- 
grees; the  openmg  of  an  angle,  being  therefore  a  portion 
of  a  circle,  must  contain  a  certain  number  of  degrees  ; 
the  larger  the  angle,  the  greater  the  number  of  degrees, 
and  the  two  angles  are  said  to  be  equal  when  they  con- 
tain an  equal  number  of  degrees. 

Emily.  Now  I  understand  it.  As  the  dimensions  of 
an  angle  depend  upon  the  number  of  degrees  contained 
between  its  lines,  it  is  the  opening  and  not  the  length  of 
its  lines,  which  determines  the  size  of  the  angle. 

Mrs.  B.  Very  well  :  now  that  you  have  a  clear  idea 
of  the  dimensions  of  angles,  can  you  tell  me  how  many 
degrees  are  contained  in  the  two  angles  formed  by  one 
line  falling  perpendicular  on  another,  as  in  the  figure  I 
have  just  drawn  1 

Emily.  You  must  allow  me  to  put  one  foot  of  the 
compasses  at  the  point  of  the  angles,  and  draw  a  circle 
round  them,  and  then  I  think  I  shall  be  able  to  answer 
your  question  :  the  two  angles  are  together  just  equal  to 
half  a  circle,  they  contain  therefore  90  degrees  each ;  90 
degrees  being  a  quarter  of  360. 

Mrs.  B.  An  angle  of  90  degrees  is  called  a  right 
angle,  and  when  one  line  is  perpendicular  to  another,  it 
forms,  you  see,  (fig.  1.)  a  right  angle  on  either  side. 
Angles  contaming  more  than  90  degrees  are  called  obtuse 
angles  (fig,  2;)  and  those  containing  less  than  90  degrees 
are  called  acute  angles,  {fig.  3.) 

Caroline.  The  angles  of  this  square  table  are  right 
angles,  but  those  of  the  octagon  table  are  obtuse  angles  ; 
and  the  angles  of  sharp-pointed  instruments  are  acute 
angles. 

Mrs.  B.  Very  well.  To  return  now  to  your  obser- 
vation, that  if  a  ball  is  thrown  obliquely  against  the  wall 
it  will  not  rebound  in  the  same  direction  ;  tell  me,  have 
you  ever  played  at  billiards  ? 

153.     Into  hAv  many  degrees  are  all  circles  divided  ? 154. 

When  are  two  angles  said  to  be  equal  ? 155.     How  many  de- 
grees are  contained  in  the  two  angles  formed  by  the  figure  named  ? 

156.     What  is  called  a  right  angle  ? — An  obtuse  angle  ? — An 

acute  angle  r 


50  ©N  THE  LAWS  OF  MOTION. 

Caroline,  Yes,  frequently  ;  and  I  have  observed  that 
when  I  push  the  ball  perpendicularly  against  the  cushion, 
it  returns  in  the  same  direction  ;  but  when  I  send  it  ob- 
liquely to  the  cushion,  it  rebounds  obliquely,  but  on  the 
opposite  side  ;  the  ball  in  this  latter  case  describes  an 
ang  e,  the  point  of  which  is  at  the  cushion.  I  have  ob- 
served too,  that  the  more  obliquely  the  ball  is  struck 
against  the  cushion,  the  more  obliquely  it  rebounds  on 
the  opposite  side,  so  that  a  billiard  player  can  calculate 
with  great  accuracy  in  what  direction  it  will  return. 

Mrs.  B.  Very  well.  This  figure  (fig.  4.  plate  II.) 
represents  a  billiard  table  ;  now  if  you  draw  a  line  A  B 
from  the  point  where  the  ball  A  strikes  perpendicular  to 
the  cushion,  you  will  find  that  it  will  divide  the  angle 
which  the  ball  describes  into  two  parts,  or  two  angles  ; 
the  one  will  show  the  obliquity  of  the  direction  of  the 
ball  in  its  passage  towards  the  cushion,  the  other  its  ob- 
liquity in  its  passage  back  from  the  cushion.  The  first 
is  called  the  angle  of  incidence,  the  other  the  angle  of  re- 
flection,  and  these  angles  are  always  equal.* 

Caroline.  This  then  is  the  reason  why,  when  I  throw 
a  ball  obliquely  against  the  wall,  it  rebounds  in  an  oppo- 
site oblique  direction,  forming  equal  angles  of  incidence 
and  of  reflection. 

Mrs.  B.  Certainly  ;  and  you  will  find  that  the  more 
obliquely  you  throw  the  ball,  the  more  obliquely  it  will 
rebound. 

We  must  now  conclude  :  but  I  shall  have  some  further 
observations  to  make  upon  the  laws  of  motion,  at  our  next 
meeting. 


*  The  Angle  of  Incidence  is  that  which  is  contained  between 
the  line  described  by  the  incident  ray,  and  a  line  perpendicular 
to  the  surface  on  which  the  ray  strikes,  raised  from  the  point  of 
incidence.  The  Angle  of  Reflection  is  that  which  is  contained 
between  the  line  described  by  the  reflected  ray,  and  a  line  per- 
pendicular to  the  reflecting  surface  at  the  point  in  which  the  in- 
cident ray  strikes  that  surface. 

157.     How  does  the   angle  of  incidence  compare,  as  to  size, 

with  the  angle  of  reflection  ? 158.     How  would*  you  illustrate 

the  angle  of  incidence  and  reflection  by  Fig.  4,  plJte  II  ^ 159. 

What  is  an  angle  of  incidence  ? 160.     What  is  an  angle  of 

reflection  ?  ^ 


ON  COMPOUND  MOTION.  51 

CONVERSATION  IV. 

ON  COMPOUND  MOTION. 

Compound  Motion^  the  Result  of  two  Opposite  Forces  ; 
Of  Circular  Motion,  the  Result  of  two  Forces,  one  of 
which  confines  the  Body  to  a  Fixed  Point ;  centre  of  Mo- 
tion, the  Point  at  Rest  while  the  other  Parts  of  the  Body 
move  round  it ;  Centre  of  Magnitude,  the  Middle  of  a 
Body  ;  Centripetal  Force,  that  which  confines  a  Body 
to  a  fixed  Central  Point ;  Centrifugcd  Force,  that  lohich 
impels  a  Body  to  fly  from  the  Centre  ;  Fall  of  Bodies  in 
a  Parabola;  Centre  of  Gravity,  the  Centre  of  Weight, 
or  point  about  which  the  Parts  balance  each  other. 

MRS.  B. 

I  MUST  now  explain  to  you  the  nature  of  compound  mo- 
tion. Let  us  suppose  a  body  to  be  struck  by  two  equal 
forces  in  opposite  directions,  how  will  it  move  ? 

Emily.  If  the  directions  of  the  forces  are  in  exact  op- 
position to  each  other,  I  suppose  the  body  would  not  move 
at  all. 

Mrs,  B.  You  are  perfectly  right ;  but  if  the  forces, 
instead  of  acting  on  the  body  in  opposition,  strike  it  in 
two  directions  inclined  to  each  other,  at  an  angle  of  nine- 
ty degrees,  if  the  ball  A  (fig.  5,  plate  II.)  be  struck  by 
equal  forces  at  X  and  at  Y,  will  it  not  move  ? 

Emily.  The  force  X  would  send  it  towards  B,  and 
the  force  Y  towards  C,  and  since  these  forces  are  equal,  I 
do  not  know  how  the  body  can  obey  one  impulse  rather 
than  the  other,  and  yet  I  think  the  ball  would  move,  be- 
cause as  the  two  forces  do  not  act  in  direct  opposition, 
they  cannot  entirely  destroy  the  effect  of  each  other. 

Mrs.  B.  Very  true  ;  the  ball  will  therefore  follow 
the  direction  of  neither  of  the  forces,  but  will  move  in  a 
line  between  them,  and  will  reach  D  in  the  same  space 
of  time  that  the  force  X  would  have  sent  it  to  B,  and  the 

162.     Of  what  does  the  fourth    Conversation   treat? 163. 

What  would  be  the  effect  if  two  bodies  were  to  strike  each  other, 

when  moving  in  opposite  directions  and  with  equal  forces  ? 

164.  What  would  be  the  effect  if  they  were  to  strike  in  directions 
inclined  to  each  other,  at  an  an;^le  of  ninety  degrees  .''—165 
How  would  you  explain  Fig  5,  plate  II. .'' 


5^  ON  COMPOUND  MOTTON^. 

force  Y  would  have  sent  it  to  C.  Now  if  you  draw  two 
lines  from  D,  to  join  B  and  C,  you  will  form  a  square, 
and  the  oblique  line  which  the  body  describes  is  called 
the  diagonal  of  the  square. 

Caroline,  That  is  very  clear,  but  supposing  the  two 
forces  to  be  unequal,  that  the  force  X,  for  instance,  be 
twice  as  great  as  the  force  Y  ? 

Mrs,  B.  Then  the  force  X  would  drive  the  ball  twice 
as  far  as  the  force  Y,  consequently  you  must  draw  the 
line  A  B  (fig.  6.,)  twice  as  long  as  the  line  A  C,  the 
body  will  in  this  case  move  to  D ;  and  if  you  draw  lines 
from  that  point  to  B  and  C,  you  will  find  that  the  ball 
has  moved  in  the  diagonal  of  a  rectangle. 

Emily,  Allow  me  to  put  another  case  ?  Suppose  the 
two  forces  are  unequal,  but  do  not  act  on  the  ball  in  the 
direction  of  a  right  angle,  but  in  that  of  an  acute  angle, 
what  will  result  ? 

Mrs.  J5.  Prolong  the  lines  in  the  directions  of  the 
iwo  forces,  and  you  will  soon  discover  which  way  the 
ball  will  be  impelled ;  it  will  move  from  A  to  D,  in  the 
diagonal  of  a  parallelogram,  (fig.  7.)  Forces  acting  in 
the  direction  of  lines  forming  an  obtuse  angle,  will  also 
produce  motion  in  the  diagonal  of  a  parallelogram.  For 
instance,  if  the  body  set  out  from  B,  instead  of  A,  and 
was  impelled  by  the  forces  X  and  Y,  it  would  move  in 
the  dotted  diagonal  B  C. 

We  may  now  proceed  to  circular  motion  :  this  is  the 
result  of  two  forces  on  a  body,  by  one  of  which  it  is  pro- 
jected forward  in  a  right  line,  whilst  by  the  other  it  is 
confined  to  a  fixed  point.  For  instance,  when  I  whirl 
this  ball,  which  is  fastened  to  my  hand  with  a  string,  the 
ball  moves  in  a  circular  direction ;  because  it  is  acted  on 
by  two  forces,  that  which  I  give  it  which  represents  the 
force  of  projection,  and  that  of  the  string  which  confines 
it  to  my  hand.  If  during  its  motion  you  were  suddenly 
to  cut  the  string,  the  ball  would  fly  off  in  a  straight  line  : 
being  released  from  confinement  to  the  fixed  point,  it 
would  be  acted  on  but  by  one  force,  and  motion  produced 
by  one  force,  you  know,  is  always  in  a  right  line. 

166.     What  is  the  oblique  hne  called,  which  is  described  by  two 

equal  forces  moving  in.  right  angular  directions? 167.     What 

does  Fig.  6,  of  that  plate  illustrate  ^ 163.     What  is  illustrated 

by  Fig.  7,  plate  IT.  ^ 169.     Of  what  is  circular  motion  the  re- 

suit .' 170.     What  simple  instance  of  circular  motion  thus  pro 

duced  could  you  give  - 


ON  COMPOUND  MOTION.  53 

Caroline,     This  is  a  little  more  difficult  to  comprehend 
than  compound  motion  in  straight  lines. 

Mrs,  B.  You  have  seen  a  mop  trundled,  and  have 
observed  that  the  threads  which  compose  the  head  of 
the  mop  fly  from  the  centre  ;  but  being  confined  to  it  at 
one  end,  they  cannot  part  from  it ;  whilst  the  water  they 
contain,  being  unconfined,  is  thrown  off  in  straight  lines. 
Emily,  In  the  same  way,  the  flyers  of  a  windmill, 
when  put  in  motion  by  the  wind,  would  be  driven 
straight  forwards  in  a  right  line,  were  they  not  confined  to 
a  fixed  point  round  which  they  are  compelled  to  move. 

Mrs.  B,  Very  well.  And  observe,  that  the  point  to 
which  the  motion  of  a  small  body,  such  as  the  ball  with 
the  string,  which  may  be  considered  as  revolving  in  one 
plane,  is  confined,  becomes  the  centre  of  its  motion. 
But  when  the  bodies  are  not  of  a  size  or  shape  to  allow 
of  oar  considering  every  part  of  them  as  moving  in  the 
same  plane,  they  in  reality  revolve  round  a  line,  which 
line  is  called  the  axis  of  motion.  In  a  top,  for  instance, 
when  spinning  on  its  point,  the  axis  is  the  line  which  passes 
through  the  middle  of  it,  perpendicularly  to  the  floor. 

Caroline,  The  axle  of  the  flyers  of  the  windmill  is 
then  the  axis  of  its  motion ;  but  is  the  centre  of  motion 
always  in  the  middle  of  a  body  1 

Mrs,  B,  No,  not  always.  The  middle  point  of  a 
body  is  called  its  centre  of  magnitude,  or  position,  that 
is,  the  centre  of  its  mass  or  bulk.  Bodies  have  also 
another  centre,  called  the  centre  of  gravity,  which  I  shall 
explain  to  you  ;  but  at  present  we  must  confine  ourselves 
to  the  axis  of  motion.  This  line  you  must  observe  re- 
mains at  rest,  whilst  all  the  other  parts  of  the  body  move 
around  it ;  when  you  spin  a  top  the  axis  is  stationary 
whilst  every  other  part  is  in  motion  round  it. 

Caroline.  But  a  top  generally  has  a  motion  forwards, 
besides  its  spinning  motion  ;  and  then  no  point  within  it 
can  be  at  rest  ? 

Mrs,  B,  What  I  say  of  the  axis  of  motion  relates 
only  to  circular  motion  ;  that  is  to  say,  to  motion  round 
a  line,  and  not  to  that  which  a  body  may  have  at  the  same 
time  in  any  other  direction.     There  is  one  circumstance 

171.     What  is  meant  by  the  axis  of  motion  ? 172.     Is  the 

centre  of  motion  always  in  the  middle  of  a  body  ? 173.     What 

is  the  middle  point  of  a  body  called  ? 174      How  is  the  ve- 
locity of  motion  at  different  distances  from  the  axis  of  motion  ? 
5* 


54  ON  COMPOUND  MOTION. 

in  circular  motion,  which  you  must  carefully  attend  to  ; 
which  is,  that  the;  further  any  part  of  a  body  is  from  the 
axis  of  motion,  the  greater  is  its  velocity  ;  as  you  approach 
that  line,  the  velocity  of  the  parts  gradually  diminish  till 
you  reach  the  axis  of  motion,  which  is  perfectly  at  rest. 

Caroline,  But,  if  every  part  of  the  same  body  did  not 
move  with  the  same  velocity,  that  part  which  moved 
quickest,  must  be  separated  from  the  rest  of  the  body,  and 
leave  it  behind  ? 

Mrs,  B.  You  perplex  yourself  by  confounding  the 
idea  of  circular  motion,  with  that  of  motion  in  a  right 
line  ;  you  must  think  only  of  the  motion  of  a  body  round 
a  fixed  line,  and  you  will  fmd,  that  if  the  parts  farthest 
from  the  centre  had  not  the  greatest  velocity,  those  parts 
would  not  be  able  to  keep  up  with  the  rest  of  the  body, 
and  would  be  left  behind.  Do  not  the  extremities  of  the 
vanes  of  a  windmill  move  over  a  much  greater  space 
than  the  parts  nearest  the  axis  of  motion  ]  (pi.  III.  fig. 
1.)  The  three  dotted  circles  describe  the  patlis  in  which 
three  different  parts  of  the  vanes  move,  and  though  the 
circles  are  of  different  dimensions,  the  vanes  describe  each 
of  them  in  the  same  space  of  time. 

Caroline.  Certainly  they  do  ;  and  I  now  only  wonder 
that  we  neither  of  us  ever  made  the  observation  before  ; 
and  the  same  effect  must  take  place  in  a  solid  body,  like 
the  top  in  spinning  ;  the  most  bulging  part  of  the  surface 
must  move  with  the  greatest  rapidity. 

Mrs,  B,  The  force  which  confines  a  body  to  a  cen- 
tre, round  which  it  moves,  is  called  the  centripetal  force ; 
and  that  force  which  impels  a  body  to  fiy  from  the  centre 
is  called  the  centrifugal  force  ;  in  circular  motion  these 
two  forces  constantly  balance  each  other ;  otherwise  the 
revolving  body  would  either  approach  the  centre,  or  re- 
cede from  it,  according  as  the  one  or  the  other  prevailed. 

Caroline,  When  I  see  any  body  moving  in  a  circle, 
I  shall  remember  that  it  is  acted  on  by  two  forces. 

Mrs,  B.  Motion,  either  in  a  circle,  an  ellipsis,  or 
any  other  curve-line,  must  be  the  result  of  the  action  of 
two  forces  ;  for  you  know,  that  the  impulse  of  one  single 
force  always  produces  motion  in  a  right  line. 

175.     What   fiorure    illustrates   this? 176.     What   are    the 

forces  called  in  circular  motion,  that  balance  or  act  in  opposition 

to  each  other? 177.     What  is  meant  by  centripetal  motion  r 

• 178.     What  is  meant  by  centrifugal  motion  ? 


ON  COMPOUND  MOTION.  65 

Emily.  And  if  any  cause  should  destroy  the  centripetal 
force,  the  centrifugal  force  would  alone  impel  the  body, 
and  it  would,  I  suppose,  fly  off  in  a  straight  line  from  the 
centre  to  which  it  had  been  confined. 

Mrs,  B,  It  would  not  fly  ofl*  in  a  right  line  from  the 
centre  ;  but  in  a  right  line  in  the  direction  in  which  it 
was  moving,  at  the  instant  of  its  release  ;  if  a  stone,  whirl- 
ed round  in  a  sling,  gets  loose  at  the  point  A  (plate  III. 
fig.  2.)  it  flies  ofl"  in  the  direction  A  B  ;  this  line  is  called 
a  tangent,  it  touches  the  circumference  of  the  circle,  and 
forms  a  right  angle  with  a  line  drawn  from  that  point  of 
the  circumference,  to  the  centre  of  the  circle  C. 

Emily,  You  say,  that  motion  in  a  curve-line  is  owing 
to  two  forces  acting  upon  a  body ;  but  when  I  throw  this 
ball  in  a  horizontal  direction,  it  describes  a  curve  line  in 
falling ;  and  yet  it  is  only  acted  upon  by  the  force  of  pro- 
jection ;  there  is  no  centripetal  force  to  confine  it,  or  pro- 
duce compound  motion. 

Mrs,  B,  A  ball  thus  thrown  is  acted  upon  by  no  less 
than  three  forces  ;  the  force  of  projection,  v/hicli  you  com- 
municated to  it ;  the  resistance  of  the  air  through  which 
it  passes,  which  diminishes  its  velocity,  without  changing 
its  direction  ;  and  the  force  of  gravity,  which  finally 
brings  it  to  the  ground.  The  power  of  gravity,  and  the 
resistance  of  the  air,  being  always  greater  than  any  force 
of  projection  we  can  give  a  body,  the  latter  is  gradually 
overcome,  and  the  body  brought  to  the  ground  ;  but  the 
stronger  the  projectile  force,  the  longer  will  these  powers 
be  in  subduing  it,  and  the  further  the  body  will  go  before 
it  falls. 

Caroline,  A  shot  fired  from  a  cannon,  for  instance, 
will  go  much  further,  than  a  stone  projected  by  the  hand. 

3Irs,  B,  Bodies  thus  projected,  you  observed,  describ- 
ed a  curve-line  in  their  descent ;  can  you  account  for 
that? 

Caroline,  No;  I  do  not  understand,  why  it  should 
not  fall  in  the  diagonal  of  a  square. 

Mrs,  B,  You  must  consider  that  the  force  of  projec- 
tion is  strongest  when  the  ball  is  first  thrown ;  this  force, 

179.     What  would  be  the  consequence,  if,  in  circular  motion, 

the  centripetal  should  be  destroyed  ? 180.     Which  figure  il- 

"  lustrates  tins  ' 181.     What  is  the  line  called  in  which  a  body 

would  fiv  ^^  'ftho  centripetal  force  were  destroyed  .'* 182.     If 

a  ball  is  thrown  horizontally,  how  many  forces  operate  upon  it-? 
163.     What  are  they  called  f* 


56  ON  COMPOUND  MOTION. 

as  it  proceeds,  being  weakened  by  the  continued  resist- 
ance of  the  air,  the  stone,  therefore,  begins  by  moving 
in  a  horizontal  direction  ;  but  as  the  stronger  powers  pre- 
vail, the  direction  of  the  ball  will  gradually  change  from 
a  horizontal  to  a  perpendicular  line.  Projection  alone 
would  drive  the  ball  A  to  B,  (fig.  3,)  gravity  would  bring 
it  to  C ;  therefore,  when  acted  on  in  different  directions, 
by  these  two  forces,  it  moves  between,  gradually  inclining 
more  and  more  to  the  force  of  gravity,  in  proportion  as 
this  accumulates  ;  instead  therefore  of  reaching  the 
ground  at  D,  as  you  supposed  it  would,  it  falls  somewhere 
about  E. 

Caroline,  It  is  precisely  so  ;  look,  Emily,  as  I  throw 
this  ball  directly  upwards,  how  the  resistance  of  the  air 
and  gravity  conquer  projection  !  Now  I  will  throw  it 
upwards  obliquely  :  see,  the  force  of  projection  enables  it, 
for  an  instant,  to  act  in  opposition  to  that  of  gravity ;  but 
it  is  soon  brought  down  again. 

Mrs.  B,  The  curve-line  which  the  ball  has  described, 
is  called  in  geometry,  di  parabola ;  but  when  the  ball  is 
thrown  perpendicularly  upwards,  it  will  descend  perpen- 
dicularly ;  because  the  force  of  projection,  and  that  of 
gravity,  are  in  the  same  line  of  direction. 

We  have  noticed  the  centres  of  magnitude,  and  of  mo- 
tion ;  but  I  have  not  yet  explained  to  you  what  is  meant 
by  the  centre  of  gravity  ;  it  is  that  point  in  a  body,  about 
which  all  the  parts  exactly  balance  each  other  ;  if,  there- 
fore, that  point  is  supported,  the  body  will  not  fall.  Do 
you  understand  this  1 

Emily,  I  think  so  ;  if  the  parts  round  about  this  point 
have  an  equal  tendency  to  fall,  they  will  be  in  equilibrium, 
and  as  long  as  this  point  is  supported,  the  body  cannot  fall. 

Mrs,  B.  Caroline,  what  would  be  the  effect,  were  any 
other  point  of  the  body  alone  supported  ? 

Caroline,  The  surrounding  parts,  no  longer  balancing 
each  other,  the  body,  I  suppose,  would  fall  on  the  side  at 
which  the  parts  are  heaviest. 

Mrs,  B,  Infallibly :  whenever  the  centre  of  gravity 
is  unsupported,  the  body  must  fall.  This  sometimes  hap- 
pens with  an  overloaded  wagon  winding  up  a  steep  hiD, 

184.     How  would  you  explain  Fig.  3.  plate  III.? 185.    What 

is  a  parabola  ? 186.     Why  will  a  stone  thrown  perpendicular- 
ly into  the  air  descend  perpendicularly  .'' 187.     vVhat  is  meant 

by  the  centre  of  gravity  : 1S8.     What  part  of  a  body  must  be 

supported  to  keep  it  from  falhng  ^ 


ON  COMPOUND  MOTION.  57 

one  side  of  the  road  being  more  elevated  than  the  other  ; 
let  us  suppose  it  to  slope  as  is  described  in  this  figure, 
(plate  III.  fig  4,)  we  will  say,  that  the  centre  of  gravity 
of  this  loaded  wagon  is  at  the  point  A.  Now  your 
eye  will  tell  you  that  a  wagon,  thus  situated,  will  over- 
set ;  and  the  reason  is,  that  the  centre  of  gravity,  A,  is 
not  supported  ;  for  if  you  draw  a  perpendicular  line  from 
it  to  the  ground  at  C,  it  does  not  fall  under  the  wagon 
within  the  wheels,  and  is  therefore  not  supported  by 
them. 

Caroline,  I  understand  that  perfectly  ;  but  what  is 
the  meaning  of  the  other  point  B  ? 

Mrs,  B.  Let  us,  in  imagination,  take  oif  the  upper 
part  of  the  load  ;  the  centre  of  gravity  will  then  change 
its  situation,  and  descend  to  B,  as  that  will  now  be  the 
point  about  which  the  parts  of  the  less  heavily  laden  wa- 
gon will  balance  each  other.  Will  the  wagon  now  be 
upset  ? 

Caroline,  No,  because  a  perpendicular  line  from  that 
point  falls  within  the  wheels  at  D,  and  is  supported  by 
them ;  and  when  the  centre  of  gravity  is  supported,  the 
body  will  not  fall. 

Emily.  Yet  I  should  not  much  like  to  pass  a  wagon 
in  that  situation  ;  for,  as  you  see,  the  point  D  is  but 
just  within  the  left  wheel  ;  if  the  right  wheel  was  merely 
raised,  by  passing  over  a  stone,  the  point  D  would  be 
thrown  on  the  outside  of  the  left  wheel,  and  the  wa- 
gon would  upset. 

Caroline,  A  wagon,  or  any  carriage  whatever,  will 
then  be  most  firmly  supported,  when  the  centre  of  gra- 
vity falls  exactly  between  the  wheels  ;  and  that  is  the  case 
in  a  level  road. 

Pray,  whereabouts  is  the  centre  of  gravity  of  the  hu- 
man body  ? 

Mrs,  B,  Between  the  hips  ;  and  as  long  as  we  stand 
upright,  this  point  is  supported  by  the  feet ;  if  you  lean 
on  one  side,  you  will  find  that  you  no  longer  stand  firm. 
A  rope-dancer  performs  all  his  feats  of  agility,  by  dexte- 
rously supporting  his  centre  of  gravity  ;  whenever  he  finds 
that  he  is  in  danger  of  losing  his  balance,  he  shifts  the 
heavy  pole,  which  he  holds  in  his  hands,  in  order  to  throw 

18^.     What  explanation  would  you  give   of  Fig.  4,  plate  III.  ? 

190.     Why  do  persons  in  ascendin?  a  hill  incline  forward,  and 

in  descending  it  incline  backward  ? 191.  How  is  it  thnt  rope- 
dancers  are  able  to  perform  their  feats  of  agility  without  falling  ? 


58  ON  COMPOUND  MOTION. 

the  weight  towards  the  side  that  is  deficient ;  and  thus  by 
changing  the  situation  of  the  centre  of  gravity,  he  restores 
his  equihbriuni. 

Caroline.  When  a  stick  is  poised  on  the  tip  of  the 
finger,  is  it  not  by  supporting  its  centre  of  gravity  ? 

3Irs.  B.  Yes  ;  and  it  is  because  the  centre  of  gravity 
is  not  supported,  that  spherical  bodies  roll  down  a  slope. 
A  sphere  being  perfectly  round,  can  touch  the  slope  but 
by  a  single  point,  and  that  point  cannot  be  perpendicularly 
under  the  centre  of  gravity,  and  therefore  cannot  be  sup- 
ported, as  you  will  perceive  by  examining  this  figure,  (fig. 
o.  plate  III.) 

Emily.  So  it  appears ;  yet  I  have  seen  a  cylinder  of 
wood  roll  up  a  slope  ;  how  is  that  contrived  ? 

Mm.  B.  It  is  done  by  plugging  one  side  of  the  cylin- 
der with  lead,  as  at  B.  (fig.  5.  plate  III.)  the  body  being 
no  longer  of  a  uniform  density,  the  centre  of  gravity  is 
removed  from  the  middle  of  the  body  to  some  point  in  the 
lead,  as  that  substance  is  much  heavier  than  wood ;  now 
you  may  observe  that  in  order  that  the  cylinder  may  roll 
down  the  plane,  as  it  is  here  situated,  the  centre  of  gra- 
vity must  rise,  which  is  impossible  ;  the  centre  of  gravity 
must  always  descend  in  moving,  and  will  descend  by  the 
nearest  and  readiest  means,  which  will  be  by  forcing  the 
cylinder  up  the  slope,  until  the  centre  of  gravity  is  sup- 
ported, and  then  it  stops. 

Caroline.  The  centre  of  gravity,  therefore,  is  not  al- 
ways in  the  middle  of  a  body. 

Mrs.  B.  No,  that  point  we  have  called  the  centre  of 
magnitude  ;  when  the  body  is  of  a  uniform  density  the 
centre  of  gravity  is  in  the  same  point ;  but  when  one  part 
of  the  body  is  composed  of  heavier  materials  than  another 
part,  the  centre  of  gravity  being  the  centre  of  the  weight 
of  the  body  can  no  longer  correspond  with  the  centre  of 
magnitude.  Thus  you  see  the  centre  of  gravity  of  this 
cylinder,  plugged  with  lead,  cannot  be  in  the  same  spot  as 
the  centre  of  magnitude. 

Emily.     Bodies,  therefore,  consisting  but  of  one  kind 

192.     Why  do  spherical  bodies  roll  down  a  slope  or  inclined 

plane  ? 193.     By    which    figure    is    this   illustrated  ? 194. 

How   can  a   cylinder    of  wood    be  made   to  roll  up    a    slope  .'* 

195.     Is  the  centre  of  gravity  always  the  centre  of  magnitude? 

190.     When  is  the  centre  of  gravity  in  the   same  point  with 

the  centre  of  magnitude? 197.     When    will   they    not  be  in 

the  same  point? 


ON  COMPOUND  MOTION.  59 

of  substance,  as  wood,  stone,  or  lead,  and  whose  densities 
are  consequently  uniform,  must  stand  more  firmly,  and 
be  more  difficult  to  overset,  than  bodies  composed  of  a 
variety  of  substances,  of  different  densities,  which  may 
throw  the  centre  of  gravity  on  one  side. 

Mrs,  B.  Yes ;  but  there  is  another  circumstance 
which  more  materially  affects  the  firmness  of  their  position, 
and  that  is  their  form.  Bodies  that  have  a  narrow  base 
are  easily  upset,  for  if  they  are  the  least  inclined,  their 
centre  is  no  longer  supported,  as  you  may  perceive  in 
fig.  6. 

Caroline,  I  have  often  observed  with  what  difficulty 
a  person  carries  a  single  pail  of  water  ;  it  is  owing,  1 
suppose,  to  the  centre  of  gravity  being  thrown  on  one  side, 
and  the  op}X)site  arm  is  stretched  out  to  endeavour  to  bring 
it  back  to  its  original  situation  ;  but  a  pail  hanging  on 
each  arm  is  carried  without  difficulty,  because  they  ba- 
lance each  other,  and  the  centre  of  gravity  remains  sup- 
ported by  the  feet. 

Mrs.  B,  Very  well ;  I  have  but  one  more  remark  to 
make  on  the  centre  of  gravity,  which  is,  that  when  two 
bodies  are  fastened  together,  by  a  line,  string,  chain,  or  any 
power  whatever,  they  are  to  be  considered  as  forming  but 
one  body  ;  if  the  two  bodies  be  of  equal  weight,  the  centre 
of  gravity  will  be  in  the  middle  of  the  line  which  unites 
them,  (fig.  7,)  but  if  one  be  heavier  than  the  other,  the 
centre  of  gravity  will  be  proportionally  nearer  the  heavy 
body  than  the  light  one.  (fig.  8.)  If  you  were  to  carry  a  rod 
or  pole  with  an  equal  weight  fastened  at  each  end  of  it, 
you  would  hold  it  in  the  middle  of  the  rod,  in  order  that 
the  weights  should  balance  each  other ;  whilst  if  it  had 
unequal  weights  at  each  end,  you  would  hold  it  nearest 
the  greater  weight,  to  make  them  balance  each  other. 

Emily,  And  in  both  cases  we  should  support  the  cen- 
tre of  gravity  ;  and  if  one  weight  be  very  considerably 
larger  than  the  other,  the  centre  of  gravity  will  be  thrown 
out  of  the  rod  into  the  heaviest  weight.      (fig.  9.) 

Mrs,  B,     Undoubtedly. 

198.     What  bodies  stand  most  firmly,  and  what  ones  are  most 

easily  upset  ? 199.     What  is  the  object  of  Fig.  6,  plate  III.  t 

200.     Why  can  a  person  carry  two  pails  of  water,  one  in  each 

hand,  easier  than  a  sinj^le  pail  ? 201.  If  two  bodies  are  connect- 
ed togetlier,  how  are  they  to  be  considered  as  to  their  centre  of  gra- 
vity ^ 20*2.     If  they  are  of  equal  weight,  where  will  the  centre  of 

gravity  be  ? 203.     If  they  are  of  unequal  weight,  where  will  it 

be  ? 204.     What  is  the  object  of  Fig.  7,  8,  and  9,  of  plate  III.  ? 


60  ON  THE  MECHANICAL  POWERS. 

CONVERSATION  V. 

ON  THE  MECHANICAL  POWERS. 

Of  the  Power  of  Machines ;  Of  the  Lever  in  General;  Of 
the  Lever  of  the  First  Kind,  having  the  Fulcrum  be- 
tween the  Power  and  the  Weight ;  Of  the  Lever  of  the 
Second  Kind,  having  the  Weight  between  the  Power 
and  the  Fulcrum ;  Of  the  Lever  of  the  Third  Kind, 
having  the  Power  beticeen  the  Fulcrum  and  the  Weight. 

MRS.  B. 

We  may  now  proceed  to  examine  the  mechanical  pow- 
ers ;  they  are  six  in  number,  one  or  more  of  which  enters 
into  the  composition  of  every  machine.  The  lever,  the 
pulley,  the  ivheel,  and  axle,  the  inclined  plane,  the  wedge, 
and  the  screiv. 

In  order  to  understand  the  power  of  a  machine,  there 
are  four  things  to  be  considered.  1st.  The  power  that 
acts  :  this  consists  in  the  effort  of  men  or  horses,  of 
weights,  springs,  steam,  &c. 

2dly.  The  resistance  which  is  to  be  overcome  by  the 
power  ;  this  is  generally  a  weight  to  be  moved.  The 
power  must  always  be  superiour  to  the  resistance,  other- 
wise the  machine  could  not  be  put  in  motion. 

Caroline,  If,  for  instance,  the  resistance  of  a  carriage 
was  greater  than  the  strength  of  the  horses  employed  to 
draw  it,  they  would  not  be  able  to  make  it  move. 

Mrs,  B,  3dly.  We  are  to  consider  the  centre  of  mo- 
tion, or  as  it  is  termed  in  mechanicks,  the  fulcrum ;  this, 
you  may  recollect,  is  the  point  about  which  all  the  parts 
of  the  body  move ;  and  lastly,  the  respective  velocities 
of  the  power,  and  of  the  resistance. 

Emily,  That  must  depend  upon  their  respective  dis- 
tances from  the  axis  of  motion ;  as  we  observed  in  the 
motion  of  the  vanes  of  the  windmill. 

Mrs,  B,  We  shall  now  examine  the  power  of  the 
lever.  The  lever  is  an  inflexible  rod  or  beam  of  any  kind, 
that  is  to  say,  one  which  will  not  bend  in  any  direction, 

205.     How  many    of  the  mechanical   powers  are    there  ?■ 

206.     What  are  the  names  of  them  ? '207.     In  order  to  un- 
derstand the  power  of  a  machine,  how    many  things  are  to  be 

considered 208.    What  is  the  first  ? — the  second  ? — the  third  ^ 

^200.     What  is  the  lever  - 


ON  THE  MECHANICAL  POWERS.  61 

For  instance,  the  steel  rod  to  which  tliese  scales  are  sus- 
pended is  a  lever,  and  the  point  in  which  it  is  supported 
the  fulcrum,  or  centre  of  motion ;  now,  can  you  tell  me 
why  the  two  scales  are  in  equilibrium  ? 

Caroline,  Being  both  empty,  and  of  the  same  weight, 
they  balance  each  other. 

Emily,  Or,  more  correctly  speaking,  because  the 
centre  of  gravity  common  to  both  is  supported. 

Mrs,  B,  Very  well  ;  and  which  is  the  centre  of  gra- 
vity of  this  pair  of  scales?  (fig.  1.  plate  IV.) 

Emily,  You  have  told  us  that  when  two  bodies  of 
equal  weight  were  fastened  together,  the  centre  of  gravity 
was  in  the  middle  of  the  line  that  connected  them  ;  the 
centre  of  gravity  of  the  scales  must  therefore  be  in  the  ful- 
crum F  of  the  lever  which  unites  the  two  scales ;  and  cor- 
responds with  the  centre  of  motion. 

Caroline,  But  if  the  scales  contained  different  weights, 
the  centre  of  gravity  would  no  longer  be  in  the  fulcrum  of 
the  lever,  but  removed  towards  that  scale  which  contained 
the  heaviest  weight ;  and  since  that  point  would  no  longer 
be  supported,  the  heavy  scale  would  descend  and  out- 
weigh the  other. 

Mrs,  B,  True  ;  but  tell  me,  can  you  imagine  any 
mode  by  which  bodies  of  different  weights  can  be  made  to 
balance  each  other,  either  in  a  pair  of  scales,  or  simply 
suspended  to  the  extremities  of  the  lever  ?  for  the  scales 
are  not  an  essential  part  of  the  machine,  they  have  no  me- 
chanical power,  and  are  used  merely  for  the  convenience 
of  containing  the  substance  to  be  weighed. 

Caroline,  What  !  make  a  light  body  balance  a  heavy 
one  ?  I  cannot  conceive  that  possible. 

3Irs,  B,  The  fulcrum  of  this  pair  of  scales  (fig.  2.)  is 
moveable,  you  see  ;  I  can  take  it  off  the  prop,  and  fasten 
it  on  again  in  another  part ;  this  part  is  now  become  the 
fulcrum,  but  it  is  no  longer  in  the  centre  of  the  lever. 

Caroline,  And  the  scales  are  no  longer  true  ;  for  that 
which  hangs  on  the  longest  side  of  the  lever  descends. 

Mrs,  B,  The  two  parts  of  the  lever  divided  by  the  ful- 
crum are  called  its  arms,  you  should  therefore  say  the 
longest  arm,  not  the  longest  side  of  the  lever.     These 

210.     Why  are  the  scales  as  seen  in  Fig.  1,  plate  IV.  in  equi- 

lifbrium  ? 211.     What  is  the  centre  of  gravity  to  two  scales  in 

e/iuilibrium  as  seen  in  that  figure  ? ^212.     What  are  the  arms 

of  a  lever  ? 

6 


Q%  ON  THE  MECHANICAL  POWERS. 

arms  are  likewise  frequently  distinguished  by  the  appella- 
tions of  the  acting  and  the  resisting  part  of  the  lever. 

Your  observation  is  true  that  the  balance  is  now  de- 
stroyed ;  but  it  will  answer  the  purpose  of  enabling  you 
to  comprehend  the  power  of  a  lever  when  the  fulcrum  is 
not  in  the  centre. 

Emily,  This  would  be  an  excellent  contrivance  for 
those  who  cheat  in  the  weight  of  their  goods ;  by  making 
the  fulcrum  a  little  on  one  side,  and  placing  the  goods  in 
the  scale  which  is  suspended  to  the  longest  arm  of  the 
lever,  they  would  appear  to  weigh  more  than  they  do  in 
reality. 

Mrs,  B,  You  do  not  consider  how  easily  the  fraud 
would  be  detected ;  for  on  the  scales  being  emptied,  they 
would  not  hang  in  equilibrium. 

Emily.  True  ;  I  did  not  think  of  that  circumstance. 
But  I  do  not  understand  why  the  longest  arm  of  the  lever 
should  not  be  in  equilibrium  with  the  other. 

Caroline,  It  is  because  it  is  heavier  than  the  shortest 
arm  ;  the  centre  of  gravity,  therefore,  is  no  longer  sup- 
ported. 

Mrs,  B,  You  are  right  ;  the  fulcrum  is  no  longer  in 
the  centre  of  gravity  ;  but  if  we  can  contrive  to  make  the 
fulcrum  in  its  present  situation  become  the  centre  of  gra- 
vity, the  scales  v/ill  again  balance  each  other  ;  for  you 
recollect  that  the  centre  of  gravity  is  that  point  about 
which  every  part  of  the  body  is  in  equilibrium. 

Emily,  It  has  just  occurred  to  me  how  this  may  be 
accomplished  ;  put  a  great  weight  into  the  scale  suspended 
to  the  shortest  arm  of  the  lever,  and  a  smaller  one  into 
that  suspended  to  the  longest  arm.  Yes,  I  have  disco- 
vered it — look,  Mrs.  B.,  the  scale  on  the  shortest  arm  will 
carry  21bs.,  and  that  on  the  longest  arm  only  one,  to  re- 
store the  balance,     (fig.  3.) 

MrSf  B,  You  see,  therefore,  that  it  is  not  so  imprac- 
ticable as  you  imagined  to  make  a  heavy  body  balance  a 
light  one ;  and  this  is  in  fact  the  means  by  which  you 
thought  an  imposition  in  the  weight  of  goods  might  be 
effected,  as  a  weight  of  ten  or  twelve  ounces  might  thus 
be  made  to  balance  a  pound  of  goods.     Let  us  now  take 

213.     What  is  the  reason  that  the  arms  of  the   lever,  as  seen 

Fig.  2,  plate  IV.  are  not  supported  ^ 214.     In  what  way  can 

Jhey  be  made  to  support  each  other  .'< 215.     What  is  illustrated 

bj  Fig.  3,  plate  IV.  ^ 


ON  THE  MECHANICAL  1»0>VERS.  63 

off  the  scales  that  we  may  consider  the  lever  simply  ;  and 
in  this  state  you  see  that  the  fulcrum  is  no  longer  the  cen- 
tre of  gravity  ;  but  it  is,  and  must  ever  be,  the  centre  of 
motion,  as  it  is  the  only  point  which  remains  at  rest, 
while  the  other  parts  move  about  it. 

Caroline,  It  now  resembles  the  two  opposite  vanes  of 
a  windmill,  and  the  fulcrum  the  point  round  which  they 
move. 

Mrs,  B,  In  describing  the  motion  of  those  vanes,  you 
may  recollect  ouf  observing  that  the  further  a  body  is 
from  the  axis  of  motion,  the  greater  is  its  velocity. 

Caroline,     That  I  remember  and  understood  perfectly. 

Mrs,  B,  You  comprehend  then,  that  the  extremity 
of  the  longest  arm  of  a  lever  must  move  with  greater 
velocity  than  that  of  the  shortest  arm  ? 

Emily,  No  doubt,  because  it  is  furthest  from  the  cen- 
tre of  motion.  And  pray,  Mrs.  B.,  when  my  brothers 
play  at  see-saw,  is  not  the  plank  on  which  they  ride  a 
kind  of  lever  ? 

Mrs,  B,  Certainly  ;  the  log  of  wood  which  supports 
it  from  the  ground  is  the  fulcrum,  and  those  who  ride 
represent  the  power  and  the  resistance  at  each  end  of 
the  lever.  And  have  you  not  observed  that  when  those 
who  ride  are  of  equal  weight,  the  plank  must  be  sup- 
ported in  the  middle  to  make  the  two  arms  equal ;  whilst 
if  the  persons  differ  in  weight,  the  plank  must  be  drawn 
a  little  further  over  the  prop,  to  make  the  arms  unequal, 
and  the  lightest  person  who  represents  the  resistance, 
must  be  placed  at  the  extremity  of  the  longest  arm. 

Caroline,  That  is  always  the  case  when  I  ride  on  a 
plank  with  my  youngest  brother  ;  I  have  observed  also 
that  the  lightest  person  has  the  best  ride,  as  he  moves 
both  further  and  quicker  ;  and  I  now  understand  that  it 
is  because  he  is  more  distant  from  the  centre  of  motion. 

Mrs,  B,  The  greater  the  velocity  with  which  your  little 
brother  moves,  renders  his  momentum  equal  to  yours. 

Caroline,  Yes  ;  I  have  the  most  gravity,  he  the  great- 
est velocity  ;  so  that  upon  the  whole  our  momentums  are 
equal.  But  you  said,  Mrs.  B.,  that  the  power  should  be 
greater  than  the  resistance  to  put  the  machine  in  motion  ; 
how  then  can  the  plank  move  if  the  momentums  of  the 
persons  who  ride  are  equal  ? 

SI 6.  What  is  the  velocity  of  the  extremity  of  the  longest  arm 
of  a  lever  compared  with  that  of  the  shortest  arm  ? 


64  ON  THE  MECHANICAL  POWERS. 

Mrs.  B,  Because  each  person  at  his  descent  touches 
the  ground  with  his  feet ;  the  re-action  of  which  gives  him 
an  impulse  which  increases  his  velocity ;  this  spring  is 
requisite  to  destroy  the  equilibrium  of  the  power  and  the 
resistance,  otherwise  the  plank  would  not  move.  Did 
you  ever  observe  that  a  lever  describes  the  arc  of  a  circle 
in  its  motion  ? 

Emily.  No  ;  it  appears  to  me  to  rise  and  descend 
perpendicularly  ;  at  least  I  always  thought  so. 

Mrs.  B.  I  believe  I  must  m.ake  a  sketch  of  you  and 
your  brother  riding  on  a  plank,  in  order  to  convince  you 
of  your  error,  (fig.  4,  pi.  IV.)  You  may  now  observe 
that  a  lever  can  move  only  round  the  fulcrum,  since  that 
is  the  centre  of  motion ;  it  would  be  impossible  for  you 
to  rise  perpendicularly  to  the  point  A,  or  for  your  brother 
to  descend  in  a  straight  line  to  the  point  B  ;  you  must 
in  rising  and  he  in  descending  describe  arcs  of  your 
respective  circles.  This  drawing  shows  you  also  how 
much  superiour  his  velocity  must  be  to  yours  ;  for  if  you 
could  swing  quite  round,  you  would  each  complete  your 
respective  circles  in  the  same  time. 

Caroline.  My  brother's  circle  being  much  the  largest, 
he  must  undoubtedly  move  the  quickest. 

Mrs.  B.  Now  tell  me,  do  you  think  that  your  brother 
could  raise  you  as  easily  without  the  aid  of  a  lever  ? 

Caroline.     Oh  no,  he  could  not  lift  me  off  the  ground. 

Mrs.  B.  Then  I  think  you  require  no  further  proof 
of  the  power  of  a  lever,  since  you  see  what  it  enables  your 
brother  to  perform. 

Caroline.  I  now  understand  what  you  meant  by  say- 
ing,  that  in  mechanicks,  motion  was  opposed  to  matter, 
for  it  is  my  brother's  velocity  which  overcomes  my  w^eight. 

Mrs.  B.  You  may  easily  imagine,  what  enormous 
weights  may  be  raised  by  levers  of  this  description,  for 
the  longer  the  acting  part  of  the  lever  in  comparison  to 
the  resisting  part,  the  greater  is  the  effect  produced  by 
it  ;  because  the  greater  is  the  velocity  of  the  power  com- 
pared to  that  of  the  weight. 

There  are  three  different  kinds  of  levers  ;  in  the  first 
the  fulcrum  is  between  the  power  and  the  weight. 

217.     What  does  a  levor  in  its  motion  describe  ? 218.     What 

is  the  design  of  Fig.^4,  plate  IV.  .? 219,     To  what  is  the  great- 
ness of  effect  produced  by  the  lever  proportional  ? 220.     How 

many  kinds  of  levers  aro  there  ? 


ON  THE  MECHANICAL  POWERS.  65 

Caroline,  This  kind  then  comprehends  the  several 
levers  you  have  described. 

Mrs,  B,  Yes,  when  in  levers  of  the  first  kind,  the  ful- 
crum is  equally  between  the  power  and  the  weight,  as  in 
the  balance  the  power  must  be  greater  than  the  weight, 
in  order  to  move  it ;  for  nothing  can  in  this  case  be 
gained  by  velocity  ;  the  two  arms  of  the  lever  being  equal, 
the  velocity  of  their  extremities  must  be  so  likewise.  The 
balance  is  therefore  of  no  assistance  as  a  mechanical 
power,  but  it  is  extremely  useful  to  estimate  the  respective 
weights  of  bodies. 

But  when  (fig.  5.)  the  fulcrum  F  of  a  lever  is  not  equally 
distant  from  the  power  and  the  weight,  and  that  the  power 
P  acts  at  the  extremity  of  the  longest  arm,  it  may  be  less 
than  the  weight  W,  its  deficiency  being  compensated  by 
its  superiour  velocity  ;  as  we  observed  in  the  see-smv, 

Emily.  Then  when  we  want  to  lift  a  great  weight, 
we  must  fasten  it  to  the  shortest  arm  of  a  lever,  and  apply 
our  strength  to  the  longest  arm  ? 

Mrs.  B.  If  the  case  will  admit  of  your  putting  the 
end  of  the  lever  under  the  weight,  no  fastening  will  be  re- 
quired ;  as  you  will  perceive  by  stirring  the  fire. 

Emily.  Oh  yes  !  the  poker  is  a  lever  of  the  first  kind, 
the  point  where  it  rests  against  the  bars  of  the  grate,  whilst 
I  am  stirring  the  fire,  is  the  fulcrum  ;  the  short  arm  or 
resisting  part  of  the  lever  is  employed  in  lifting  the 
weight,  which  is  the  coals,  and  my  hand  is  the  power  ap- 
plied to  the  longest  arm,  or  Acting  part  of  the  lever. 

Mrs.  B.  Let  me  hear,  Caroline,  whether  you  can 
equally  well  explain  this  instrument,  which  is  composed 
of  two  levers,  united  in  one  common  fulcrum. 

Caroline.     A  pair  of  scissors  ! 

Mrs.  B.  You  are  surprised,  but  if  you  examine  their 
construction,  you  will  discover  that  it  is  the  power  df  the 
lever  that  assists  us  in  cutting  with  scissors. 

Caroline.  Yes ;  I  now  perceive  that  the  point  at 
which  the  two  levers  are  screwed  together,  is  the  fulcrum  ; 
the  handles,  to  which  the  power  of  the  fingers  is  applied, 


221.     Where    is  the  fulcrum   in  the  first  kind  ? 222.     How 

are  we  to  use  levers  of  the  first  kind  in  raisins^  large  weights  ? 

223.     What  power  of  mechanicks  do  the  common  scissors  involve  .'' 

224.     How  may  the  scissors  be  explained  as  formed  by  the 

lever  ="  €  * 


66  ©W  THE  MECtfANlCAL  POWERS. 

are  the  extremities  of  the  acting  part  of  the  leversy  and 
the  cutting  part  of  the  scissors,  are  the  resisting  parts  of 
the  levers  :  therefore,  the  longer  the  handles  and  the 
shorter  the  points  of  the  scissors,  the  more  easily  you  cut 
with  them. 

Emily,  That  I  have  often  observed,  for  when  I  cut 
pasteboard  or  any  hard  substance,  I  always  make  use  of 
that  part  of  the  scissors  nearest  the  screw  or  rivet,  and 
I  now  understand  why  it  increases  the  power  of  cutting  ; 
but  I  confess  I  never  should  have  discovered  scissors  to 
have  been  double  levers  ;  and  pray  are  not  snuffers 
levers  of  a  similar  description  ? 

Mrs.  B,  Yes,  and  most  kinds  of  pincers ;  the  great 
power  of  which  consists  in  the  resisting  part  of  the  lever 
being  very  short  in  comparison  of  the  acting  part. 

Caroline,  And  of  what  nature  are  the  two  other  kinds 
of  levers  ? 

Mrs.  B,  In  levers  of  the  second  kind,  the  weight, 
instead  of  being  at  one  end,  is  situated  between  the  powef 
and  the  fulcrum,     (fig.  6.) 

Caroline,  The  weight  and  the  fulcrum  have  here 
changed  places  ;  and  what  advantage  is  gained  by  this 
kind  of  lever  ? 

Mrs,  B,  In  moving  it,  the  velocity  of  the  power  must 
necessarily  be  greater  than  that  of  the  weight,  as  it  is  more 
distant  from  the  centre  of  the  motion. 

Have  you  ever  seen  your  brother  move  a  snow-ball  by 
means  of  a  strong  stick,  when  it  became  too  heavy  for 
him  to  move  without  assistance  1 

Caroline,  Oh  yes  ;  and  this  was  a  lever  of  the  second 
order  (fig.  7.)  ;  the  end  of  the  stick,  which  he  thrusts 
under  the  ball,  and  which  rests  on  the  ground,  becomes 
the  fulcrum  ;  the  ball  is  the  weight  to  be  moved,  and  the 
power  his  hands  applied  to  the  other  end  of  the  lever* 
In  this  instance  there  is  an  immense  difference  in  the 
length  of  the  arms  of  the  kver  ;  for  the  weight  is  almost 
close  to  the  fulcrum. 

Mrs.  B,  And  the  advantage  gained  is  proportional 
to  this  difference.  Fishermen's  boats  are  by  levers  of 
this   description  raised  from  the  ground  to  be  launched 

225.     How   is  the    second  kind    of  lever  designated  ? 226* 

Wliich  figures  illustrate  the  use  of  levers  of  the  second  kind? 
-^ — 227.  To  what  is  the  advantage  gained  in  the  use  of  the  se- 
cond kind  of  lever  proportional  ? 


ON  THE  MECHANICAL  POWERS.  67 

into  the  sea,  by  means  of  slippery  pieces  of  board  whicli 
are  thrust  under  the  keel.  The  most  common  example 
that  we  have  of  levers  of  the  second  kind  is  in  the  doors 
of  our  apartments. 

Emily,  The  hinges  represent  the  fulcrum,  our  hands 
the  power  applied  to  the  other  end  of  the  lever  ;  but 
where  is  the  weight  to  be  moved  ? 

Mrs.  B.  The  door  is  the  weight,  and  it  consequently 
occupies  the  whole  of  the  space  between  the  power  and 
the  fulcrum.  Nut-crackers  are  double  levers  of  this 
kind  ;  the  hinge  is  the  fulcrum,  the  nut  the  resistance, 
and  the  hands  the  power. 

In  levers  of  the  third  kind,  (fig.  8.),  the  fulcrum  is  again 
at  one  of  the  extremities,  the  weight  or  resistance  at  the 
other,  and  it  is  now  the  power  which  is  applied  between 
the  fulcrum  and  the  resistance. 

Emily,  The  fulcrum,  the  weight,  and  the  power,  then, 
each  in  their  turn,  occupy  some  part  of  the  middle  of  the 
lever  between  its  extremities.  But  in  this  third  kind  of 
lever,  the  weight  being  further  from  the  centre  of  motion 
than  the  power,  the  difficulty  of  raising  it  seems  increased 
rather  than  diminished. 

Mrs.  B.  That  is  very  true  ;  a  lever  of  this  kind  is 
therefore  never  used,  unless  absolutely  necessary,  as  is 
the  case  in  lifting  up  a  ladder  perpendicularly  in  order  to 
place  it  against  the  wall ;  the  man  who  raises  it  cannot 
place  his  hands  on  the  upper  part  of  the  ladder,  the  power, 
therefore,  is  necessarily  placed  much  nearer  the  fulcrum 
than  the  weight. 

Caroline.  Yes,  the  hands  are  the  power,  the  ground 
the  fulcrum,  and  the  upper  part  of  the  ladder  the  weight. 

Mrs.  B.  Nature  employs  this  kind  of  lever  in  the 
structure  of  the  human  frame.  In  lifting  a  weight  with 
the  hand,  the  lower  part  of  the  arm  becomes  a  lever  of 
the  tinrd  kind  :  the  elbow  is  the  fulcrum,  the  muscles  of 
the  fleshy  part  of  the  arm  the  power  ;  and  as  these  are 
nearer  to  the  elbow  than  the  hand,  it  is  necessary  that 
their  power  should  exceed  the  weight  to  be  raised. 

228.  What  are  the  most  common  examples  of  levers  of  the  se- 
cond kind  ? 229.     How  would  you  explain  the  opening  of  a 

common  door,  as  involving  the  principle  of  the  second  kind  of  le- 
vers ? 230.     What  is  the  third  kind  of  levers  ? 231.     What 

is  an  instance   of  its  use  ? 232.    'How  does  the  raising  of  a 

weight  by  the  hand  represent  this  kind  of  levers .'' 


68  OS  THE  MECHANICAL  POWERS. 

Emily.  Is  it  not  surprising  that  nature  should  have 
furnished  us  with  such  disadvantageous  levers. 

Mrs.  B.  The  disadvantage,  in  respect  to  power,  is 
more  than  counterbalanced  by  the  convenience  resulting 
from  this  structure  of  the  arm  :  and  it  is  no  doubt  that 
which  is  best  adapted  to  enable  it  to  perform  its  various 
functions. 

We  have  dwelt  so  long  on  the  lever,  that  we  must  re- 
serve the  examination  of  the  other  mechanical  powers  to 
our  next  interview. 


CONVERSATION  V. 

CONTINUED. 

ON  THE  MECHANICAL  POWERS. 

Of  the  Pulley  ;   Of  the  Wheel  and  Axle ;  Of  the  Inclined 
Plane ;  Of  the  Wedge  ;  Of  the  Screw, 

MRS.  B. 

The  pulley  is  the  second  mechanical  power  we  are  to 
examine.     You  both,  I  suppose,  have  seen  a  pulley  ? 

Caroline.  Yes.  frequently  :  it  is  a  circular  and  flat 
piece  of  wood  or  metal,  with  a  string  which  runs  in  a 
groove  round  it  ;  by  means  of  which,  a  weight  may  be 
pulled  up  ;  thus  pulleys  are  used  for  drawing  up  curtains. 

Mrs.  B.  Yes ;  but  in  that  instance  the  pulleys  are 
fixed,  and  do  not  increase  the  power  to  raise  the  weights, 
as  you  will  perceive  by  this  figure,  (pi.  V.  fig.  1.)  Observe 
that  the  fixed  pulley  is  on  the  same  principle  as  the  lever 
of  a  pair  of  scales,  in  which  the  fulcrum  F  being  in  the 
centre  of  gravity,  the  power  P  and  the  weight  W,  are 
equally  distant  from  it,  and  no  advantage  is  gained. 

Emily.  Certainly  ;  if  P  represents  the  power  employ- 
ed to  raise  the  weight  W,  the  power  must  be  greater  than 
the  weight  in  order  to  move  it.  But  of  what  use  then  are 
pulleys  in  mechanicks  ? 

233.     What  is  the  second  mechanical   power  ? 234.     What 

is  a  pulley  ? 235.     How  does  Fig.  1.  plate  V.  illustrate  the  fixed 

pulley  ^ 236.     How  must  the  power  compare  with  the  weight 

in  order  to  move  it,  by  the  use  of  the  fixed  pulley  ? 


ON  THE  MECHANICAL  POWERS.  W 

Mrs.  B.  The  next  figure  represents  a  pulley  which  is 
not  fixed,  (fig»  2.)  and  thus  situated  you  will  perceive  that 
it  aflfords  us  mechanical  assistance.  In  order  to  raise  the 
weight  (W)  one  inch,  P,  the  power,  must  draw  the  strings 
B  and  C  one  inch  each :  the  whole  string  is  therefore 
shortened  two  inches,  while  the  weight  is  raised  only  one. 

Emily,  That  I  understand  :  if  P  drew  the  string  but 
one  inch,  the  weight  would  be  raised  only  half  an  inch, 
because  it  would  shorten  the  strings  B  and  C  half  an  inch 
each,  and  consequently  the  pulley,  with  the  weight  at- 
tached to  it,  can  be  raised  only  half  an  inch. 

Caroline.  I  am  ashamed  of  my  stupidity ;  but  I  con- 
fess that  I  do  not  understand  this  ;  it  appears  to  me  that 
the  weight  would  be  raised  as  much  as  the  string  is  short- 
ened by  the  power. 

Mrs,  B.  I  will  endeavour  to  explain  it  more  clearly. 
I  fasten  this  string  to  a  chair,  and  draw  it  towards  me ;  I 
have  now  shortened  the  string,  by  the  act  of  drawing  it, 
one  yard. 

Caroline,  And  the  chair,  as  I  supposed,  has  advanced 
one  yard. 

Mrs,  B,  This  exemplifies  the  nature  of  a  single  fixed 
pulley  only.  Now  unfawSten  the  string,  and  replace  the 
chair  where  it  stood  before.  In  order  to  represent  the 
moveable  pulley,  we  must  draw  the  chair  forwards  by  put- 
ting the  string  round  it ;  one  end  of  the  string  may  be  fas« 
tened  to  the  leg  of  the  table,  and  I  shall  draw  the  chair  by 
the  other  end  of  the  string.  I  have  again  shortened 
the  string  one  yard  ;  how  much  has  the  chair  advanced  ? 

Caroline,  I  now  understand  it ;  the  chair  represents 
the  weight  to  which  the  moveable  pulley  is  attached  ; 
and  it  is  very  clear  that  the  weight  can  be  drawn  only 
half  the  length  you  draw  the  string.  I  believe  the  cir- 
cumstance that  perplexed  me  was.  that  I  did  not  observe 
the  difference  that  results  from  the  v/eight  being  attached 
to  the  pulley,  instead  of  being  fastened  to  the  string,  as 
is  the  case  in  the  fixed  pulley. 

Emily,  But  I  do  not  yet  understand  the  advantage  of 
pulleys  ;  they  seem  to  me  to  increase  rather  than  diminish 
the  difhculty  of  raising  weights,  since  you  must  draw  the 
string  double  the  length  that  you  raise  the  weight ;  whilst 

237.  What  kind  of  pulley  does  Fig.  2,  plate  V.  represent,  and 
hov/  would  you  explain  it  ? 


70  ON  THE  MECHANICAL  POWERS. 

with  a  single  pulley,  or  without  any  pulley,  the  weight  k 
raised  as  much  as  the  string  is  shortened. 

Mrs,  B.  The  advantage  of  a  moveable  pulley  consists 
in  dividing  the  difficulty  ;  we  must  draw,  it  is  true,  twice 
tlie  length  of  the  string,  but  then  only  half  the  strength 
is  reqiiired  that  would  be  necessary  to  raise  the  weight 
without  the  assistance  of  a  moveable  pulley. 

Emily,  So  that  the  difficulty  is  overcome  in  the  same 
manner  as  it  would  be,  by  dividing  the  weight  into  two 
equal  parts,  and  raising  them  successively. 

Mrs,  B,  Exactly.  You  must  observe,  that  with  a 
moveable  pulley  the  velocity  of  the  power  is  double  that  of 
the  weight,  since  the  power  P  (fig.  2.)  moves  two  inches, 
whilst  the  weight  W  moves  one  inch ;  therefore  the 
power  need  not  be  more  than  half  the  weight  to  make 
their  momentums  equal. 

Caroline.  Pulleys  act  then  on  the  same  principle  as 
the  lever,  the  deficiency  of  strength  of  the  power  being 
compensated  by  its  superiour  velocity. 

Mrs.  B.  You  will  find  that  all  mechanical  power  is 
founded  on  the  same  principle. 

Emily.  But  may  it  not  be  objected  to  pulleys,  that  a 
longer  time  is  required  to  raise  a  weight  by  their  aid  than 
without  it ;  for  what  you  gain  in  power  you  lose  in  time  1 

Mrs.  B.  Tliat,  my  dear,  is  the  fundamental  law  in 
mechanicks  :  it  is  the  case  with  the  lever  as  well  as  the 
pulley ;  and  you  will  find  it  to  be  so  with  all  the  other 
mechanical  powers. 

Caroline.  I  do  not  see  any  advantage  in  the  mecha- 
nical powers  then,  if  what  we  gain  by  them  one  way  is  lost 
another. 

Mrs.  B.  Since  we  are  not  able  to  increase  our  natu- 
ral strength,  is  not  that  science  of  wonderful  utility,  by 
means  of  which  we  may  reduce  the  resistance  or  weight 
of  any  body  to  the  level  of  our  strength  ?  This  the 
mechanical  powers  enable  us  to  accomplish,  by  dividing 
the  resistance  of  a  body  into  parts  which  we  can  succes- 

238.  In  what  does  the  advantage  of  a  moveable  pulley  consist  ? 
239.  How  do  the  weight  and  power  of  a  moveable  pulley  com- 
pare, that  their  momenta  be  equal  ? 240.     On  what  principle 

are   all   mechanical  powers  founded.' 241.     Is  there  any  loss 

of  time  in  the  use  of  the  moveable  pulley  ? 242.     And  to  what 

is  this  loss  of  time  proportional  ? 243.  What  then  is  the  ad- 
vantage of  this  pulley,  or  of  any  of  the  mechanical  powers,  if  there 
Lsas  much  loss  in  time  as  gain  in  power  r 


ON  THE  MECHANICAL  POWERS-  71 

aively  overcome.  It  is  true,  as  you  observe,  that  it 
requires  a  sacrifice  of  time  to  attain  this  end,  but  you 
must  be  sensible  how  very  advantageously  it  is  exchanged 
for  power ;  the  utmost  exertion  we  can  make  adds  but 
little  to  our  natural  strength,  whilst  we  have  a  much 
more  unlimited  command  of  time.  You  can  now  under- 
stand, that  the  greater  the  number  of  pulleys  connected 
by  a  string,  the  more  easily  the  weight  is  raised,  as  the 
difficulty  is  divided  among  the  number  of  strings,  or  rather 
of  parts  into  which  the  string  is  divided  by  the  pulleys. 
Several  pulleys  thus  connected,  form  what  is  called  a  sys- 
tem, or  tackle  of  pulleys,  (fig.  3.)  You  may  have  seen 
them  suspended  from  cranes  to  raise  goods  into  ware- 
houses, and  in  ships  to  draw  up  the  sails. 

Emily,  But  since  a  fixed  pulley  affords  us  no  mecha- 
nical  aid,  why  is  it  ever  used  ? 

Mrs,  B.  Though  it  does  not  increase  our  power,  k 
is  frequently  useful  for  altering  its  direction.  A  single 
pulley  enables  us  to  draw  up  a  curtain,  by  drawing  down 
the  string  connected  with  it  ;  and  we  should  be  much  at 
a  loss  to  accomplish  this  simple  operation  without  its  as- 
sistance. 

Caroline,  There  would  certainly  be  some  difficulty 
in  ascending  to  the  head  of  the  curtain,  in  order  to  draw 
it  up.  Indeed,  I  now  recollect  having  seen  workmen 
raise  small  weights  by  this  means,  which  seemed  to  an- 
swer a  very  useful  purpose, 

Mrs,  B.  In  shipping,  both  the  advantages  of  an  in- 
crease of  power  and  a  change  of  direction,  by  means  of 
pulleys,  are  united  :  for  the  sails  are  raised  up  the  masts 
by  the  sailors  on  deck,  from  the  change  of  direction  which 
the  pulley  effects,  and  the  labour  is  facilitated  by  the  me- 
chanical power  of  a  combination  of  pulleys. 

Emily,  But  the  pulleys  on  ship-board  do  not  appear 
to  me  to  be  united  in  the  manner  you  have  shown  us. 

Mrs,  B.  They  are,  I  believe,  generally  connected  as 
described  in  figure  4,  both  for  nautical,  and  a  variety  of 
other  purposes  ;  but  in  whatever  manner  pulleys  are  con- 
nected by  a  single  string,  the  mechanical  power  is  the 
same. 


244.     What  is  a  system  or  tackle  of  pulleys,  and  which  fi^^ure 

exhibits  it  ? 245.     If  there  is  no  mechanical  aid  from  the  fixed 

pulley,  why  is  it  used  ? 


7/J  ON  THE  MECHANICAL  POWERS. 

The  third  mechanical  power  is  the  wheel  and  axle. 
Let  us  suppose  (plate  V.  fig.  5.)  the  weight  W  to  be  a 
bucket  of  water  in  a  well,  which  we  raise  by  winding  the 
rope,  to  which  it  is  attached,  round  the  axle  :  if  this  be 
done  without  a  wheel  to  turn  the  axle,  no  mechanical  as- 
sistance is  received. 

The  axle  without  a  wheel  is  as  impotent  as  a  single 
fixed  pulley,  or  a  lever,  whose  fulcrum  is  in  the  centre ; 
but  add  the  wheel  to  the  axle,  and  you  will  immediately 
find  the  bucket  is  raised  with  much  less  difficulty. 

The  velocity  of  the  circumference  of  the  wheel  is  as 
much  greater  than  that  of  the  axle,  as  it  is  further  from  the 
centre  of  motion  :  for  the  wheel  describes  a  great  circle 
in  the  same  space  of  time  that  the  axle  describes  a  small 
one,  therefore  the  power  is  increased  in  the  same  pro- 
portion as  the  circumference  of  the  wheel  is  greater  than 
that  of  the  axle.  If  the  velocity  of  the  wheel  is  twelve 
times  greater  than  that  of  the  axle,  a  power  nearly  twelve 
times  less  than  the  weight  of  the  bucket  would  be  able  to 
raise  it. 

Emily,  The  axle  acts  the  part  of  the  shorter  arm  of 
the  lever,  the  wheel  that  of  the  longer  arm. 

Caroline,  In  raising  water  there  is  commonly,  I  be- 
lieve, instead  of  a  wheel  attached  to  the  axle,  only  a 
crooked  handle,  which  answers  the  purpose  of  winding 
the  rope  round  the  axle,  and  thus  raising  the  bucket. 

Mrs.  J5.  In  this  manner  (fig.  6.) ;  now  if  you  observe 
the  dotted  circle  which  the  handle  describes  in  winding 
up  the  rope,  you  will  perceive  that  the  branch  of  the  han- 
dle A,  which  is  united  to  the  axle,  represents  the  spoke 
of  a  wheel,  and  answers  the  purpose  of  an  entire  wheel ; 
the  other  branch  B  affords  no  mechanical  aid,  merely 
serving  as  a  handle  to  turn  the  wheel. 

Wheels  are  a  very  essential  part  to  most  machines  : 
they  are  employed  in  various  ways  ;  but,  when  fixed  to 
the  axle,  their  mechanical  power  is  always  the  same  ; 
that  is,  as  the  circumference  of  the  wheel  exceeds  that  of  the 
axle,  so  much  will  the  energy  of  its  power  be  increased. 

Caroline,  Then  the  larger  the  wheel  the  greater  must 
be  its  effect. 

246.     What   is  the  third    mechanical   power?- 247.     What 

^oes  Fig.  5,  plate  V.  illustrate  .' 248.     In  what  proportion  is  the 

power  of  the  wheel  increased  ^ 249.     How  may  a  wheel  he 

compared  to  the  lever  ? 250.     How  does  Fig.  6,  plate  V.  repre- 

fient  a  wheel  ' 


ON  THE  MECHANICAL  POWERS.  73 

Mrs.  B,  Certainly.  If  you  have  ever  seen  any  con- 
siderable mills  or  manufactures,  you  must  have  admired 
the  immense  wheel,  the  revolution  of  which  puts  the 
whole  of  the  machinery  into  motion  ;  and  though  so  great 
an  effect  is  produced  by  it,  a  horse  or  two  has  sufficient 
power  to  turn  it ;  sometimes  a  stream  of  water  is  used  for 
that  purpose,  but  of  late  years,  a  steam-engine  has  been 
found  both  the  most  povverful  and  the  most  convenient 
mode  of  turning  the  wheel. 

Caroline,  Do  not  the  vanes  of  a  windmill  represent  a 
wheel,  Mrs.  B.  ? 

3Irs*  J5.  Yes ;  and  in  this  instance  we  have  the  ad- 
vantage of  a  gratuitous  force,  the  wind,  to  turn  the 
wheel.  One  of  the  great  benefits  resulting  from  the  use 
of  machinery  is,  that  it  gives  us  a  sort  of  empire  over  the 
powers  of  nature,  and  enables  us  to  make  them  perform 
the  labour  which  would  otherwise  fall  to  the  lot  of  man. 
When  a  current  of  wind,  a  stream  of  water,  or  the  ex- 
pansive force  of  steam,  performs  our  task,  we  have  only 
to  superintend  and  regulate  their  operations. 

The  fourth  mechanical  power  is  the  inclined  plane  ; 
this  is  nothing  more  than  a  slope,  or  declivity,  frequently 
used  to  facilitate  the  drawing  up  of  weights.  It  is  not 
difficult  to  understand,  that  a  weight  may  much  more 
easily  be  drawn  up  a  slope  than  it  can  be  raised  the  same 
height  perpendicularly.  But  in  this,  as  well  as  the  other 
mechanical  powers,  the  facility  is  purchased  by  a  loss  of 
time,  (fig.  7.) ;  for  the  weight,  instead  of  moving  directly 
from  A  to  C,  m.ust  move  from  B  to  C,  and  as  the  length 
of  the  plane  is  to  its  height,  so  much  is  the  resistance  of 
the  weight  diminished. 

Emily.  Yes  ;  for  the  resistance,  instead  of  being  con- 
fined to  the  short  line  A  C,  is  spread  over  the  long  line 
BC. 

Mrs.  B.  The  wedge,  which  is  the  next  mechanical 
power,  is  composed  of  two  inclined  planes,  (fig.  8.)  :  you 

251.     On  what  mechanical  force  is  the  wind-mill  operated  ? 

252.     What  is    f-^und  to  be  the  most   powerful  and  convenient 

mode  of  turning  the  wheel  ? 253.     What  is  one  of  the  great 

benefits  resultins:  from  tho   use  of  machinery  ? 254.     What  is 

the   fourth    mechanical    power.' 2.'5.      What   is    an    inclined 

plane  ? 256.     How  would  you  explain  Fig.  7,  plate  V.  .' 

257.  How  much  is  the  resistance  of  the  weight  dimin^'shed  by  the 
use  of  the  inclined  plane  ? 258.  Of  what  is  the  wed^e  com- 
posed ."* 

7 


T4  ON  THE  MECHANICAL  POWERS. 

may  have  seen  wood-cutters  use  it  to  cleave  wood.  The 
resistance  consists  in  the  cohesive  attraction  of  the  wood, 
.or  any  other  body  which  the  v/edge  is  employed  to  sepa- 
rate ;  and  the  advantage  gained  by  this  power  is  in  the 
proportion  of  half  its  width  to  its  length  ;  for  while  the 
wedge  forces  asunder  the  coherent  particles  of  the  wood 
to  A  and  B,  it  penetrates  downwards  as  far  as  C. 

Emily.  The  wedge,  then,  is  rather  a  compound  than 
a  distinct  mechanical  power,  since  it  is  composed  of  two 
inclined  planes. 

Mrs,  13.  It  is  so.  All  cutting  instruments  are  con- 
structed upon  the  principle  of  the  inclined  plane,  or  the 
wedge  :  those  that  have  but  one  edge  sloped,  like  the 
chisel,  may  be  referred  to  the  inclined  plane  ;  whilst  the 
axe,  the  hatchet,  and  the  knife  (when  used  to  split  asun- 
der) are  used  as  wedges. 

Carolme.  But  a  knife  cuts  best  when  it  is  drawn  across 
the  substance  it  is  to  divide.  We  use  it  thus  in  cutting 
meat,  we  do  not  chop  it  to  pieces. 

3Irs,  13.  The  reason  of  this  is,  that  the  edge  of  a 
knife  is  really  a  very  fine  saw,  and  therefore  acts  best 
when  used  like  that  instrument. 

The  screw,  which  is  the  last  mechanical  power,  is  more 
complicated  than  the  others.  You  will  see  by  this  figure, 
(fig.  9.)  that  it  is  composed  of  two  parts,  the  screw  and  the 
nut.  The  screw  S  is  a  cylinder,  with  a  spiral  protuberance 
coiled  round  it,  called  the  tliread  ;  the  nut  N  is  perforated 
to  contain  the  screw,  and  the  inside  of  the  nut  has  a 
spiral  groove  made  to  fit  the  spiral  thread  of  the  screw. 

Caroline.  It  is  just  like  this  little  box,  the  lid  of  which 
screws  on  the  box  as  you  have  described  ;  but  what  is 
this  handle  which  projects  from  the  nut  ? 

3Irs.  B.  It  is  a  lever,  which  is  attached  to  the  nut, 
without  which  the  screw  is  never  used  as  a  mechanical 
power  :  the  nut  with  a  lever  L  attached  to  it  is  commonly 
called  a  winch. 

The  power  of  the  screw,  complicated  as  it  appears,  is 
referrible  to  one  of  the  most  simple  of  the  mechanical 
powers ;  which  of  them  do  you  think  it  is  ? 

259.     In  what  does   the  resistance  of  the   wedge  consist  .•' 

260.     On  what  mechanical  principles  are  cutting  instruments  de- 
signed .? 261.     Whicli  is  the  last  mechanical  power  ? 262. 

Of  what  is  the  screw  composed  ? 263.     What  is  the  construc- 
tion of  the  screw  and  nut  ? 264.     How  would  vou  explain  Fig. 

9,  plate  V.  ? 


ON  THE  MECHANICAL  POWERS.  75 

Carolhie,  In  appearance,  it  most  resembles  the  wheel 
and  axle. 

3Irs.  B,  The  lever,  it  is  true,  has  the  effect  of  a 
wlieel,  as  it  is  the  means  by  which  you  wind  the  nut 
round;  but  the  lever  is  not  considered  as  composing  a 
part  of  the  screvv,  though  it  is  true,  that  it  is  necessarily 
attached  to  it.  But  observe,  that  tho  lever,  considered  as 
a  u'heel,  is  not  fastened  to  the  axle  or  screw,  but  moves 
round  it,  and  in  so  doing,  the  nut  either  rises  or  descends, 
according  to  the  uay  in  which  you  turn  it. 

Emihj,  The  spiral  thrend  of  the  screw  resembles,  I 
think,  an  inclined  piaffe :  it  is  a  sort  of  slope,  by  means 
of  v/hich  the  nut  ascends  more  easily  than  it  would  do 
if  raised  perpendicularly  ;  and  it  serves  to  support  it 
when  at  rest. 

Mrs,  B,  Very  well  :  if  you  cut  a  slip  of  paper  in  the 
form  of  an  inclined  plane,  and  wind  it  round  your  pencil,, 
which  will  represent  the  cylinder,  you  will  find  that  it 
makes  a  spiral  line,  corresponding  to  the  spiral  protu- 
berance of  the  screw,  (fig.  10.) 

Emily.  Very  true  ;  the  nut  then  ascends  an  inclined 
plane,  but  ascends  it  in  a  spiral,  instead  of  a  straight  line  ; 
the  closer  the  thread  of  the  screw,  the  more  easy  the  as- 
cent ;  it  is  like  having  shallow,  instead  of  steep  steps  to 
ascend. 

Mrs,  B,  Yes,  excepting  that  the  nut  takes  no  steps, 
it  gradually  winds  up  or  down  ;.  then  observe,  that  the  clo- 
ser the  threads  of  the  screw,  the  greater  the  number  of 
revolutions  the  winch  must  make  ;  so  that  v^^e  return  to  the 
old  principle, — what  is  saved  in  power  is  lost  in  time. 

Emily,  Cannot  the  power  of  a  screw  be  increased 
also,  by  lengthening  the  lever  attached  to  the  nut  ? 

Mrs.  B.  Certainly.  The  screw,  with  the  addition  of 
the  lever,  forms  a  very  powerful  machine,  employed  either 
for  compression,  or  to  raise  heavy  weights.  It  is  used  by 
book-binders,  to  press  the  leaves  of  books  together ;  it  is 
used  also  in  cider  and  wine  presses,  in  coining,  and  for  a 
variety  of  other  purposes. 

All  machines  are  composed  of  one  or  more  of  these  six 
mechanical  powers  we  have  examined  :  I  have  but  one 


265.     To  which  of  the  other  mechanical  powers  is  the  screw 

referrible  r ^(5(),      How  can    the  power    of  the  screw  be  in- 

creased  ? 


76  ON  THE  MECHANICAL  POWERS. 

more  remark  to  make  to  you  relative  to  them,  which  is, 
that  friction  in  a  considerable  degree  diminishes  their 
force,  allowance  must  therefore  always  be  made  for  it  in 
the  construction  of  machinery. 

Caroline.  Ey  friction,  do  you  mean  one  part  of  the 
machine  rubbing  against  another  pari  contiguous  to  it  ? 

Mrs,  B,  Yes  ;  friction  is  the  resistance  which  bodies 
meet  with  in  rubbing  against  each  other  ;  there  is  no  such 
thing  as  perfect  smoothness  or  evenness  in  nature  :  polished 
metals,  though  they  wear  that  appearance  more  than  any 
other  bodies,  are  far  from  really  possessing  it ;  and  their 
inequalities  may  frequently  be  perceived  through  a  good 
magnifying  glass.  When,  therefore,  the  surfaces  of  the 
two  bodies  come  into  contact,  the  prominent  parts  of  the 
one  w  ill  often  fall  into  the  hollow  parts  of  the  other,  and 
occasion  more  or  less  resistance  to  motion. 

Caroline.  But  if  a  machine  is  made  of  polished  metal, 
as  a  watch,  for  instance,  the  friction  must  be  very  trifling  J 

Mrs.  JB.  In  proportion  as  the  surfaces  of  bodies  are 
well  polished,  the  friction  is  doubtless  diminished ;  but  it 
is  always  considerable,  and  it  is  usually  computed  to  de- 
stroy one-third  of  the  power  of  a  machine.  Oil  or  grease  is 
used  to  lessen  friction ;  it  acts  as  a  polish  by  tilling  up  the 
cavities  of  the  rubbing  surfaces,  and  thus  making  them 
slide  more  easily  over  each  other. 

Caroline.  Is  it  for  this  reason  that  wheels  are  greased, 
and  the  locks  and  hinges  of  doors  oiled  ? 

Mrs.  B.  Yes ;  in  these  instances  the  contact  of  the 
rubbing  surfaces  is  so  close,  and  the  rubbing  so  continual, 
that-not withstanding  their  being  polished  and  oiled,  a  con- 
siderable degree  of  friction  is  produced. 

There  are  two  kinds  of  friction ;  the  one  occasioned 
by  the  sliding  of  the  flat  surface  of  a  body,  the  other  by 
the  rolling  of  a  circular  body ;  the  friction  resulting  from 
the  first  is  much  the  most  considerable,  for  great  fcwce 
is    required    to    enable    the    sliding    body    to   overcome 

266.      What   diminishes    the     force    of   all    machinery  ? 

667.     What  are  we  to  understand  by  friction  in  machinery  ? 

268.     In  what  proportion  is  the  friction  of  machinery  destroyed  ? 

^269.     How  much  of  the  power  of  a  machine  is  reckoned  ta 

be  destroyed  by  friction  ? 270.     What   is  commonly  used  to 

lessen  the  friction  of  machinery  ? 271 .     Why  will  oil  and  grease 

lessen  the  friction  of  machinery  ^ ^272.     How  many  kinds  of 

friction  are  there  ? ^273.     What  are  they  .^ 274.     Which  is 

the  most  considerable  .' 


ON  tflfi  MECHANICAL  POWERS.        ^^  77 

the  resistance  which  the  asperities  of  the  surfaces  in  con- 
tact oppose  to  its  motion,  and  it  must  be  either  lifted  over 
or  break  through  them  ;  wiiilst,  in  the  other  kind  of  fric- 
tion, the  rough  parts  roll  over  each  other  with  comparative 
facility ;  hence  it  is,  that  wheels  are  often  used  for  the 
sole  purpose  of  diminishing  the  resistance  of  friction. 

Eniilji,  This  is  one  of  the  advantages  of  carriage- 
wheels  ;  is  it  not  ? 

3Irs,  B.  Yes ;  and  the  larger  the  circumference  of 
the  wheel,  the  more  readily  it  can  overcome  any  consider- 
able obstacles,  such  as  stones,  or  inequalities  in  the  road. 
When,  in  descending  a  steep  hill,  we  fasten  one  of  the 
wheels,  we  decrease  the  velocity  of  the  carriage,  by  in- 
creasing the  friction. 

Caroline,  That  is  to  say,  by  converting  the  rolling  fric- 
tion into  the  dragging  friction.  And  when  you  had  casters 
put  to  the  legs  of  the  table,  in  order  to  move  it  more  easily, 
you  changed  the  dragging  into  tlie  rolling  friction. 

Mrs.  B,  There  is  another  circumstance  which  we 
have  already  noticed,  as  diminishing  the  motion  of  bodies, 
and  v/hich  greatly  affects  the  power  of  machines.  This 
is  the  resistance  of  the  medium  in  which  a  machine  is 
worked.  All  fluids,  whether  of  the  nature  of  air,  or  of 
water,  are  called  mediums ;  and  their  resistance  is  pro- 
portioned to  their  density  ;  for  the  more  matter  a  body 
contains,  the  greater  the  resistance  it  will  oppose  to  the 
motion  of  another  body  striking  against  it . 

Emih/.  It  would  then  be  much  more  difficult  to  work 
a  machine  under  water  than  in  the  air  ? 

Mrs,  B.  Certainly,  if  a  machine  could  be  worked  in 
vacuo,  and  without  friction,  it  would  be  perfect ;  but  this 
is  unattainable :  a  considerable  reduction  of  power  must 
therefore  be  allowed  for  the  resistance  of  the  air. 

We  shall  here  conclude  our  observations  on  the  me- 
chanical powers.  At  our  next  meeting  I  shall  endeavour 
to  give  you  an  explanation  of  the  motion  of  the  heavenly 
bodies. 


275.     Which  will  most  readily  overcome  obstacles,  a  large  or  a 

small  wheel  ? 276.     Why  is  a  wheel   fastened  on  descending 

a  hill  ? 277.     What  besides  friction  diminishes  the  force  of  all 

machinery  ? ^278.       What  is  meant  by  mediums  ? 279.    To 

what  is  their  resistance   proportioned  ? 280.     In  what   state 

would  the  force  of  machinery  be  perfect  ? 

•7* 


78  CAUSES  OF  THE  EARTH's  ANNUALr  MOTION. 

CONVERSATION  VI. 

CAUSES  OF  THE  EARTH's  ANNUAL  MOTION. 

Of  the  Planets,  and  their  Motion ;   Of  the  Diurnal  Mo- 
tion of  the  Earth  and  Planets, 

CAROLINE. 

I  AM  come  to  you  to-day  quite  elated  with  the  spirit  of 
opposition,  Mrs.  B.  ;  for  I  have  discovered  such  a  pow- 
erful objection  to  your  theory  of  attraction,  that  I  doubt 
whether  even  your  conjuror  Newton,  with  his  magick 
wand  of  attraction,  will  be  able  to  dispel  it. 

Mrs,  B.  Well,  my  dear,  pray  what  is  this  weighty 
objection  ? 

Caroline.  You  say  that  bodies  attract  in  proportion  to 
the  quantity  of  matter  they  contain  :  now  we  all  know  the 
sun  to  be  much  larger  than  the  earth ;  why,  therefore, 
does  it  not  attract  the  earth ;  you  will  not,  I  suppose,  pre- 
tend to  say  that  we  are  falling  towards  the  sun  ? 

Emily,  However  j)lausible  your  objection  appears, 
Caroline,  I  think  you  place  too  much  reliance  upon  it : 
when  any  one  has  given  such  convincing  proofs  of  saga- 
city and  wisdom  as  Sir  Isaac  Newton,  when  we  find  that 
his  opinions  are  universally  received  and  adopted,  is  it  to 
be  expected  that  any  objection  we  can  advance  should 
overturn  them  1 

Caroline,  Yet  I  confess  that  I  am  not  inclined  to 
yield  implicit  faith  even  to  opinions  of  the  great  Newton : 
for  what  purpose  are  we  endowed  with  reason,  if  we  are 
denied  the  privilege  of  making  use  of  it,  by  judging  for 
ourselves  ? 

Mrs,  B,  It  is  reason  itself  which  teaches  us,  that 
when  we,  novices  in  science,  start  objections  to  theories 
established  by  men  of  acknowledged  wisdom,  we  should 
be  diffident  rather  of  our  own  than  of  their  opinion.  I  am 
far  from  wishing  to  lay  the  least  restraint  on  your  ques- 
tions ;  you  cannot  be  better  convinced  of  the  truth  of  a 
system,  than  by  finding  that  it  resists  all  your  attacks ; 
but  I  would   advise  you  not  to  advance  your  objections 

281.  If  bvodies  attract  eachjother  in  proportion  to  the  quantity 
of  matter  they  contain,  why  does  not  the  sun  attract  the  earth 
completely  to  itself? 


CAUSES  OF  THE  EARTH's  ANNtJAL  MOTION.  79 

with  so  much  confidence,  in  order  that  the  discovery  of 
iheir  fallacy  may  be  attended  with  less  mortification. 
In  answer  to  that  you  have  just  proposed,  I  can  only  say, 
that  the  earth  really  is  attracted  by  the  sun. 

Caroline.  Take  care  at  least  that  we  are  not  consum- 
ed by  him,  Mrs.  B. 

Mrs.  B.  We  are  in  no  danger  ;  but  our  magician 
Newton,  as  you  are  pleased  to  call  him,  cannot  extricate 
himself  from  this  difficulty  without  the  aid  of  some  caba- 
listical  figures,  which  I  must  draw  for  him. 

Let  us  suppose  the  earth,  at  its  creation,  to  have  been 
projected  forwards  into  universal  space  :  we  know  that 
if  no  obstacle  impeded  its  course,  it  would  proceed  in  the 
same  direction,  and  with  a  uniform  velocity  for  ever.  In 
fig.  1,  plate  6.,  A  represents  the  earth,  and  S  the  sun. 
We  shall  suppose  the  earth  to  be  arrived  at  the  point  in 
which  it  is  represented  in  the  figure,  having  a  velocity 
which  would  carry  it  on  to  B  in  the  space  of  one  month ; 
whilst  the  sun's  attraction  would  bring  it  to  C  in  the  same 
space  of  time.  Observe  that  the  two  forces  of  projection 
and  attraction  do  not  act  in  opposition,  but  perpendicu- 
larly, or  at  a  right  angle  to  each  other.  Can  you  tell  me 
now,  how  the  earth  will  move  ? 

Emily.  I  recollect  your  teaching  us  that  a  body  act- 
ed upon  by  two  forces  perpendicular  to  each  other  w^ould 
move  in  the  diagonal  of  a  parallelogram ;  if,  therefore,  I 
complete  the  parallelogram  by  drawing  the  lines  C  D, 
B  D,  the  earth  will  move  in  the  diagonal  A  D. 

Mrs.  B.  A  ball  struck  by  two  forces  acting  perpen- 
dicularly to  each  other,  it  is  true,  moves  in  the  diagonal 
of  a  parallelogram  ;  but  you  must  observe  that  the  force  of 
attraction  is  continually  acting  upon  our  terrestrial  ball, 
and  producing  an  incessant  deviation  from  its  course  in 
a  right  line,  which  converts  it  into  that  of  a  curve  line ; 
every  point  of  which  may  be  considered  as  constituting 
the  diagonal  of  an  infinitely  small  parallelogram. 

282.  If  the  earth  at  its  creation  had  been  put  in  motion  by  a 
single  force  without  resistance,  what  would  have  been  its  course  ? 

— - — 283.     How  would  you  illustrate  this  by  the   figure  ? 284. 

What  prevents  the  earth  from  proceeding  on  in  a  right  line,  as  im- 
pelled by  its  projectile  force  ? 285.     In  what  direction  does  the 

attractix)n  of  the  sun  operate  on  the  projectile  force  of  the  earth  ? 

286       When  two   forces    operate   perpendicularly    on    each 

other,  in  what  direction  will  be  their  compound  motion  ^ 287. 

Why  then  is  the  line  A  D  in  Figure  1,  circular  instead  of  being  a 
right  line  diagonal  to  the  parallelogram,  A  B  D  C  ? 


80  CAUSES  OF  THE  EARTH's  ANNUAL  MOTION. 

Let  us  detain  the  earth  a  moment  at  the  point  D,  and 
consider  how:  it  will  be  affected  by  the  combined  action  of 
the  two  forces  in  its  new  situation.  It  still  retains  its  ten- 
dency to  fly  off  in  a  straight  line ;  but  a  straight  line 
would  now  carry  it  away  to  F,  whilst  the  sun  would  at» 
tract  it  in  the  direction  D  S ;  how  then  will  it  proceed  ? 

Emily,  It  will  go  on  in  a  cuive  line,  in  a  direction 
between  that  of  the  two  forces. 

Mrs,  JB,  In  order  to  know  exactly  what  course  the 
earth  will  follow,  draw  another  parallelogram  similar  to 
the  first,  in  which  the  line  D  F  describes  the  force  of  pro- 
jection, and  the  line  D  S,  that  of  attraction  ;  and  you  will 
find  that  the  earth  will  proceed  in  the  curve  line  D  G. 

Caroline,  You  must  now  allow  me  to  draw  a  parallel- 
ogram, Mrs.  B.  Let  me  consider  in  what  direction  will 
the  force  of  projection  now  impel  the  earth. 

3Irs.  B,  First  draw  a  line  from  the  earth  to  the  sun 
representing  the  force  of  attraction :  then  describe  the 
force  of  projection  at  a  right  angle  to  it. 

Caroline,  The  earth  will  then  move  in  the  curve  G  I, 
of  the  parallelogram  G  H  I  K. 

Mrs,  B,  You  recollect  that  a  body  acted  upon  by 
two  forces,  moves  through  a  diagonal  in  the  same  time 
that  it  would  have  moved  through  one  of  the  sides  of  the 
parallelogram,  were  it  acted  upon  by  one  force  only. 

The  earth  has  passed  through  the  diagonals  of  these 
three  parallelograms  in  the  space  of  three  months,  and 
has  performed  one  quarter  of  a  circle  ;  and  on  the  same 
principle  it  will  go  on  till  it  has  completed  the  whole  of 
the  circle.  It  will  then  recommence  a  course,  which  it 
has  pursued  ever  since  it  first  issued  from  the  hand  of  its 
Creator,  and  which  there  is  every  reason  to  suppose  it 
will  continue  to  follow,  as  long  as  it  remains  in  existence. 

Emily,  What  a  grand  and  beautiful  effect  resulting 
from  so  simple  a  cause  ! 

Caroline,  It  affords  an  example  on  a  magnificent 
scale,  of  the  circular  motion  which  you  taught  us  in 
mechanicks.  The  attraction  of  the  sun  is  the  centripetal 
force,  which  confines  the  earth  to  a  centre  ;  and  the  im- 


288.     How  would  yon  explain  the  continued  motion  of  the  earth 

about  the  sun  by  the.  use  of  Fig.  1,  plate  VI  ^ 289.     What  is  the 

attraction  of  the  sun  called  .' 290.     And  what  is  the  projectile 

force  of  the  earth  called  P 


CAUSES  OF  THE  EARTh's  ANNUAL  MOTION.  81 

pulse  of  projection  the  centrifugal  force,  which  impels 
the  earth  to  quit  the  sun  and  fly  off  in  a  tangent. 

Mrs,  B,  Exactly  so.  A  simple  mode  of  illustrating 
the  effect  of  these  combined  forces  on  the  earth,  is  to  cut 
a  slip  of  card  in  the  form  of  a  right  angle,  (fig.  2,  plate 
VI.)  to  describe  a  small  circle  at  the  angular  point  re- 
presenting the  earth,  and  to  fasten  the  extremity  of  one  of 
the  legs  of  the  angle  to  a  fixed  point,  which  we  shall  con- 
sider as  the  sun.  Thus  situated,  the  angle  will  represent 
both  the  centrifugal  and  centripetal  forces ;  and  if  you 
draw  it  round  the  fixed  point,  you  will  see  how  the  di- 
rection of  the  centrifugal  force  varies,  constantly  forming 
a  tangent  to  tlie  circle  in  which  the  earth  moves,  as  it  is 
constantly  at  a  right  angle  with  the  centripetal  force. 

Emily,  The  earth,  then,  gravitates  towards  the  sun 
without  the  slightest  danger  either  of  approaching  nearer 
or  receding  further  from  it.  How  admirably  this  is  con- 
trived !  If  the  two  forces  which  produce  this  circular  mo- 
tion had  not  been  so  accurately  adjusted,  one  would  ulti- 
mately have  prevailed  over  the  other,  and  we  should  either 
have  approached  so  near  the  sun  as  to  have  been  burnt, 
or  have  receded  so  far  from  it  as  to  have  been  frozen. 

Mrs,  B,  What  will  you  say,  my  dear,  when  I  tell 
you  that  these  two  forces  are  not,  in  fact,  so  proportion- 
ed as  to  produce  circular  motion  in  the  earth  1 

Caroline,  You  must  explain  to  us,  at  least,  in  what 
manner  we  avoid  the  threatened  destruction. 

Mrs,  B,  Let  us  suppose  that  when  the  earth  is  at 
A.  (fig.  3.),  its  projectile  force  should  not  have  given  it 
a  velocity  sufficient  to  counterbalance  that  of  gravity,  so 
as  to  enable  these  powers  conjointly  to  carry  it  round  the 
sun  in  a  circle  ;  the  earth,  instead  of  describing  the  line 
A  C,  as  in  the  former  figure,  will  approach  nearer  the  sun 
in  the  line  A  B. 

Caroline,  Under  these  circumstances,  I  see  not  what 
is  to  prevent  our  approaching  nearer  and  nearer  the  sun 
till  we  fall  into  it :  for  its  attraction  increases  as  we  ad- 
vance towards  it,  and  produces  an  accelerated  velocity  in 
the  earth,  which  increases  the  danger. 

291.  What  simple  illustration  is  given  in  Fig.  2,  plate  VI.  of 
the  combined  forces,  which  produced  the  revolution  of  the  earth 
about  the  sun  .'' 292.  Does  the  earth  revolve  in  an  exact  cir- 
cle about  the  sun  ? 293.     What  is  the  design  of  Fig.  3,  plate 

VI.  .' 294.     In  Fig.  3,  plate  VI.  why  is  the  earth  in  the  line  at 

B  instead  of  the  line  at  C  according  to  the  principle  of  Fig.  I.  ^ 


82  CAUSES  OF  THE  EARTH's  ANNUAL  MOTION. 

Mrs,  jB.  And  there  is  yet  another  danger,  of  which 
you  are  not  aware.  Observe,  that  as  the  earth  approaches 
the  sun,  the  direction  of  its  projectile  force  is  no  longer  per- 
pendicular to  that  of  attraction,  but  inclines  more  nearly 
to  it.  When  the  earth  reaches  that  part  of  its  orbit  at  B, 
the  force  of  projection  would  carry  it  to  D,  which  brings 
it  nearer  the  sun  instead  of  bearing  it  away  from  it. 

Emily.  If,  then,  we  are  driven  by  one  power  and 
drawn  by  the  ether  to  this  centre  of  destruction,  how  is 
it  possible  for  us  to  escape  ? 

Mrs,  B.  A  little  patience,  and  you  will  find  that  we 
are  not  without  resource.  The  earth  continu^s^pproach- 
ing  the  sun  with  a  uniformly  increasing  accelerated  mo- 
tion, till  it  reaches  the  point  E.  In  what  direction  will 
the  projectile  force  now  impel  it  ? 

Emily.  In  the  direction  E  F.  Here  then  the  two  forces 
act  perpendicularly  to  each  other,  and  the  earth  is  situat- 
ed just  as  it  was  in  the  preceding  figure  ;  therefore,  from 
this  point,  it  should  revolve  round  the  sun  in  a  circle. 

Mrs.  B.  No,  all  the  circumstances  do  not  agree. 
In  motion  round  a  centre,  you  recollect  that  the  centri- 
fugal force  increases  with  the  velocity  of  the  body,  or,  in 
other  words,  the  quicker  it  moves,  the  stronger  is  its  ten- 
dency to  fly  off  in  a  right  line.  When  the  earth,  there- 
fore, arrives  at  E,  its  accelerated  motion  will  have  so 
far  increased  its  velocity,  and  consequently  its  centrifugal 
force,  that  the  latter  will  prevail  over  the  force  of  at- 
traction, and  drag  the  earth  away  fi"om  the  sun  till  it 
reaches  G. 

Caroline.  It  is  thus,  then,  that  we  escape  from  the 
dangerous  vicinity  of  the  sun  ;  and  in  proportion  as  w^e 
recede  from  it,  the  force  of  its  attraction,  and,  conse- 
quently, the  velocity  of  the  earth's  motion  are  dimi- 
nished. 

Mrs.  B.  Yes.  From  G  the  direction  of  projection  is 
towards  H,  that  of  attraction  towards  S,  and  the  earth 
proceeds  between  them  with  a  uniformly  retarded  motion, 
till  it  has  completed  its  revolution.  Thus  you  see,  that 
the  earth  travels  round  the  sun,  not  in  a  circle,  but  an 


205.     When  the  erirth  arrives  at    E  in  the  figure,  why  does  it 
not  revolve  in  a  small  circular  orbit  instead  of  recedin*^  off  in  tlie 

direction  G  ? 2*-0.     What  is  the  figure  called  that  the  earth  t'e- 

.•icrites  in  its  revolut.'on  about  the  sun  - 


CAUSES  OF  THE  EARTH's  ANNUAL  MOTION.  83 

ellipsis,  of  which  the  sun  occjpies  one  of  the  foci;  and 
that  in  its  coarse  the  earth  alternately  approaches,  and 
recedes  from  it,  without  any  danger  of  being  either  swal- 
lowed up,  or  being  eiitirely  carried  away  from  it. 

Caroline,  And  I  observe,  that  what  I  apprehended 
to  be  a  dangerous  irregularity,  is  the  means  by  which  the 
most  perfect  order  and  harmony  are  produced  ! 

Emily,  The  earth  travels,  then,  at  a  very  unequal 
rate,  its  velocity  being  accelerated  as  it  approaches  the 
sun,  and  retarded  as  it  recedes  from  it. 

Mrc^.  B.  It  is  mathematically  demonstrable,  that,  in 
moving  round  a  point  towards  v/hich  it  is  attracted,  a  body 
passes  over  equal  areas  in  equal  times.  The  whole  of  the 
space  contained  within  the  earth's  orbit,  is  in  fig,  4.,  di- 
vided into  a  number  of  areas,  or  spaces,  1,  2,  3,  4,  6lc,  all 
of  which  are  of  equal  dimensions,  though  of  very  different 
forms  ;  some  of  them,  you  see,  are  long  and  narrow,  others 
broad  an.l  short :  but  they  each  of  them  contain  an  equal 
quantity  of  space.  An  imaginary  line  drawn  from  the  cen- 
tre of  tlie  earth  to  that  of  the  sun,  and  keeping  pace  with 
the  earth  in  its  revolution,  passes  over  equal  areas  in  equal 
times  ;  that  is  to  say,  if  it  is  a  month  going  from  A  to  B,  it 
will  be  a  month  going  from  B  to  C,  and  another  from  C 
to  E,  and  so  on. 

Caroline.  What  long  journeys  the  earth  has  to  per- 
form in  tlie  course  of  a  month,  in  one  part  of  her  orbit, 
and  how  short  they  are  in  the  other  part  ! 

Mrs,  B,  The  inequality  is  not  so  considerable  as  ap- 
pears in  this  figure ;  for  the  earth's  orbit  is  not  so  eccen- 
trick  as  it  is  there  described  ;  and,  in  reality  differs  but 
little  from  a  circle  ;  that  part  of  the  earth's  orbit  nearest  the 
sun  is  called  its  Perihelion,  that  part  most  distant  from  the 
sun  its  Aphelion ;  and  the  earth  is  above  three  millions  of 
miles  nearer  the  sun  at  its  perihelion  than  at  its  aphelion. 


297.     What  is  the  name  of  the  place  occupied  by  the  sun  with- 
in the  orbit  of  the  earth  ? 298.     Is  the  earth's  motion  in  moving 

round  the  sun  uniform  ? 299.     What  is  mathematically  demon- 
strable in  relation  to  abodv  moving  round  a  point  towards  which  it 

is  attracted  ? 300.    What  is  the  desis^n  of  Fiff.  4,  plate  VI.  ? 

301 .     What  is  that  part  of  the  earth's  orbit  called  which  is  most  dis- 
tant from  th3  sun.' 302.     What   is  that  part   called  which  is 

nearest  the  sun  .' 303.    How  much  nearer  is  the  earth  to  the  sun 

in  perihehon  than  at  its  aphelion  .' 


84  CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION. 

Emily,  I  think  I  can  trace  a  consequence  from  these 
different  situations  of  the  earth  ;  is  it  not  the  cause  of 
summer  and  winter  ? 

Mrs.  B.  On  the  contrary  ;  during  the  height  of  sum- 
mer, the  earth  is  in  that  part  of  its  orbit  which  is  moFt 
distant  from  the  sun,  and  it  is  during  the  severity  of  win- 
ter, that  it  approaches  nearest  to  it. 

Emily,  That  is  very  extraordinary  ;  and  how  then 
do  you  account  for  the  heat  being  greatest,  when  we  are 
most  distant  from  the  sun  ? 

3Irs.  B.  The  difference  of  the  earth's  distance  from 
the  sun  in  summer  and  winter,\vhen  compared  with  its  total 
distance  from  the  sun,  is  but  inconsiderable.  The  earth,  it 
is  true,  is  above  three  millions  of  miles  nearer  the  sun  in 
winter  than  in  summer  ;  but  that  distance,  however  great  it 
at  first  appears,  sinks  into  insignificance  in  comparison  of 
95  millions  of  miles,  which  is  our  mean  distance  from  the 
sun.  The  change  of  temperature,  arising  from  this  diffe- 
rence, would  scarcely  be  sensible,  w^ere  it  not  completely 
overpowered  by  other  causes  which  produce  the  variations 
of  the  seasons ;  but  these  I  shall  defer  explaining  till  we  liave 
made   some  further  observations  on  the  heavenly  bodies. 

Caroline.  And  should  not  the  sun  appear  smaller  in 
summer,  when  it  is  so  much  further  from  us  ? 

3Irs.  B.  It  actually  does  when  accurately  measured  ; 
but  the  apparent  difference  in  size,  is,  I  believe,  not  per- 
ceptible to  the  n?ked  eye. 

Emily.  Then,  since  the  earth  moves  with  the  greatest 
velocity  in  that  part  of  its  orbit  nearest  the  sun,  it  must 
have  completed  its  journey  through  one  half  of  its  orbit  in 
a  shorter  time  than  the  other  half? 

3frs.  B.  Yes,  it  is  about  seven  days  longer  perform- 
ing the  summer-half  of  its  orbit,  than  the  winter-holf. 
The  revolution  of  all  the  planets  round  the  sun  is  the  re- 
sult of  the  same  causes,  and  is  performed  in  the  same 
manner  as  that  of  the  earth. 

Caroline.     Fray  what  are  the  planets  ? 

3Irs.  B.  They  are  those  celestial  bodies,  which  re- 
volve like  our  earth  about  the  sun  ;  they  are  supposed  to 
resemble  the  earth  also  in  many  other  respects  ;  and  we 

304.     Is  the  earth  nearest  tlie  sun  in  summer  or^Yinte^  ? 305. 

How  much  Jono-PT  is  the  earth  performing  the  snmnier-half  than 
the  winter -hah  of  its  orbit  ? 306.     What  are  the  planets  ? 


CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION.  85 

are  led  by  analogy  to  suppose  them  to  be  inhabited 
worlds. 

Caroline.  I  have  heard  so  ;  but  do  you  not  think  such 
an  opinion  too  great  a  stretch  of  the  imagination  1 

Mrs,  B,  Some  of  the  planets  are  proved  to  be  larger 
than  the  earth  ;  it  is  only  their  immense  distance  from  us, 
which  renders  their  apparent  dimensions  so  small.  Now, 
if  we  consider  them  as  enormous  globes,  instead  of  small 
twinkling  spots,  we  shall  be  led  to  suppose,  that  the  Al- 
mighty would  not  have  created  them  merely  for  the  pur- 
pose of  giving  us  a  little  light  in  the  night,  as  it  was 
formerly  imagined,  and  we  should  find  it  more  consistent 
with  our  ideas  of  the  Divine  wisdom  and  beneficence  to 
suppose  that  these  celestial  bodies  should  be  created  for 
the  habitation  of  beings,  who  are,  like  us,  blessed  by  his 
providence.  Both  in  a  moral  as  well  as  a  physical  point 
of  view,  it  appears  to  me  more  rational  to  consider  the 
planets  as  worlds  revolving  round  the  sun ;  and  the  fixed 
stars  as  other  suns,  each  of  them  attended  by  their  re- 
spective system  of  planets,  to  which  they  impart  their  in- 
fluence. We  have  brought  our  telescopes  to  such  a  de- 
gree of  perfection,  that  from  the  appearances  which  the 
moon  exhibits  when  seen  through  them,  we  have  very 
good  reason  to  con  hide,  that  it  is  a  habitable  globe,  for 
though  it  is  true,  that  we  cannot  discern  its  towns  and 
people,  we  can  plainly  perceive  its  mountains  and  val- 
leys ;  and  some  astronomers  have  gone  so  far  as  to  ima- 
gine they  discovered  volcanoes. 

Emily,  If  the  fixed  stars  are  suns,  with  planets  re- 
volving round  them,  why  should  we  not  see  those  planets 
as  well  as  their  suns  ? 

Mrs,  B,  In  the  first  place,  we  conclude  that  the 
planets  of  other  systems,  (like  those  of  our  own,)  are  much 
smaller  than  the  suns  which  give  them  light ;  therefore 
at  so  great  a  distance  as  to  make  the  suns  appear  like 
fixed  stars,  the  planets  would  be  quite  invisible.  Second- 
ly, the  light  of  the  planets  being  only  reflected  light,  is 
much  more  feeble  than  that  of  the  fixed  stars.  There  is 
exactly  the  same  difference   as  between  the  light  of  the 

307.     Why  do  we  suppose  the   planets  are  inhabited  ? 308. 

If  the  planets  are  worlds  like  our  own,  why  do  they  appear  so 
small  ? 309.  If  the  fixed  stars  are  suns,  with  planets  revolv- 
ing round  them,  why  should  we  not  see  those  planets  as  well  as 
their  suns  ? 

8 


86  CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION. 

sun  and  that  of  the  moon  ;  the  first  being  a  fixed  star,  the 
second  a  planet. 

Emily,  But  if  the  planets  are  worlds  like  our  earth, 
they  are  dark  bodies  ;  and  instead  of  shining  by  night, 
we  should  see  them  only  by  day-light.  And  why  do  we 
not  see  the  fixed  stars  also  by  day-light  ? 

Mrs.  B,  Both  for  the  same  reason  ;  their  light  is  so 
faint,  compared  to  that  of  our  sun  reflected  by  the  atmo- 
sphere, that  it  is  entirely  effaced  by  it ;  the  light  emitted 
by  the  fixed  stars  may  probably  be  as  strong  as  that  of  our 
sun,  at  an  equal  distance  ;  but  being  so  much  more  remote, 
it  is  diffused  over  a  greater  space,  and  is  consequently 
proportionally  weakened. 

Caroline.  True  ;  I  can  see  much  better  by  the  light  of  a 
candle  that  is  near  me,than  by  that  of  one  at  a  great  distance. 
But  I  do  not  understand  what  makes  the  planets  shine  ? 

Mrs.  B.  What  is  it  that  makes  the  steel  buttons  on 
your  brother's  coat  shine  1 

Caroline.  The  sun.  But  if  it  was  the  sun  which 
made  the  planets  shine,  we  should  see  them  in  the  day- 
time when  the  sun  shone  upon  them ;  or  if  the  faintness 
of  their  light  prevented  our  seeing  them  in  the  day,  we 
should  not  see  them  at  all,  for  the  sun  cannot  shine  upon 
them  in  the  night. 

Mrs.  B.  There  you  are  in  error.  But  in  order  to 
explain  this  to  you,  I  must  first  make  you  acquainted  with 
the  various  motions  of  the  planets. 

You  know,  that  according  to  the  laws  of  attraction,  the 
planets  belonging  to  our  system  all  gravitate  towards  the 
sun  ;  and  that  this  force  combined  with  that  of  projection, 
will  occasion  their  revolution  round  the  sun,  in  orbits  more 
or  less  elliptical,  according  to  the  proportion  which  these 
two  forces  bear  to  each  other. 

But  the  planets  have  also  another  motion  ;  they  re- 
volve upon  their  axes.  The  axis  of  a  planet  is  an  ima- 
ginary line  which  passes  through  its  centre,  and  on  which 
it  turns ;  and  it  is  this  motion  which  produces  day  and 
night.  With  that  side  of  the  planet  facing  the  sun  it  is 
day ;  and  with  the  opposite  side,  which  remains  in  dark- 
ness, it  is  night.  Our  earth,  which  we  consider  as  a 
planet,  is  24  hours  in  performing  one  revolution  on  its 

310.     Why  do  we  not  see  the  stars  in  the  daytime^ oil. 

What  motion  have  the  planets  besides  that  about  the  sun  ? 311,* 

What  is  the  axis  of  a  planet  ? 


CAUSES  OF  THE  EARTH's  ANNUAL  MOTION.  87 

axis  ;  in  that  period  of  time,  therefore,  we  have  a  day  and 
a  night ;  hence  this  revolution  is  called  the  earth's  diur- 
nal or  daily  motion  ;  and  it  is  this  revolution  of  the  earth 
from  west  to  east  which  produces  an  apparent  motion  of 
the  sun,  moon,  and  stars  in  a  contrary  direction. 

Let  us  now  suppose  ourselves  to  be  beings  independ- 
ent of  any  planet,  travelling  in  the  skies,  and  looking  up- 
on the  earth  in  the  same  point  of  view  as  upon  the  other 
planets. 

Caroline,  It  is  not  flattering  to  us,  its  inhabitants,  to 
see  it  make  so  insignificant  an  appearance. 

Mrs,  B.  To  those  who  are  accustomed  to  contem- 
plate it  in  this  light,  it  never  appears  more  glorious. 
We  are  taught  by  science  to  distrust  appearances  :  and 
instead  of  considering  the  planets  as  little  stars,  we  look 
upon  them  either  as  brilliant  suns  or  habitable  worlds, 
and  we  consider  the  whole  together  as  forming  one  vast 
and  magnificent  system,  worthy  of  the  Divine  hand  by 
which  it  was  created. 

Emily,  I  can  scarcely  conceive  the  idea  of  this  im- 
mensity of  creation  ;  it  seems  too  sublime  for  our  ima- 
gination : — and  to  think  that  the  goodness  of  Providence 
extends  over  millions  of  worlds  throughout  a  boundless 
universe — Ah  !  Mrs.  B.,  it  is  we  only  who  become  trifling 
and  insignificant  beings  in  so  magnificent  a  creation. 

Mrs,  B.  This  idea  should  teach  uis  humility,  but  with- 
c^ut  producing  despondency.  The  same  Almighty  hand 
which  guides  these  countless  worlds  in  their  undeviating 
course,  conducts  with  equal  perfection  the  blood  as  it  cir- 
culates through  the  veins  of  a  fly,  and  opens  the  eye  of 
the  insect  to  behold  His  wonders.  Notwithstanding  this 
immense  scale  of  creation,  therefore,  we  need  not  fear  to 
be  disregarded  or  forgotten. 

But  to  return  to  our  station  in  the  skies.  We  were, 
if  you  recollect,  viewing  the  earth  at  a  great  distance,  in 
appearance  a  little  star,  one  side  illuminated  by  the  sun, 
the  other  in  obscurity.  But  would  you  believe  it,  Ca- 
roline, many  of  the  inhabitants  of  this  little  star  imagine 
that  when  that  part  which  they  inhabit  is  turned  from  the 
sun,  darkness  prevails  throughout  the  universe   merely 


31o.     What  are  we  taught  by  science  ? 314.     If  the  planets 

are  only  seen  by  the  reflected  light  of  the  sun,  how  is  it  that  they 
can  be  seen  in  the  night  ? 


88  CAtJSES  OF  THE  EARTh's  ANNUAL  M<5tI0N. 

because  it  is  night  with  them ;  whilst,  in  reality,  the  sun 
never  ceases  to  shine  upon  every  planet.  When,  there- 
fore, these  little  ignorant  beings  look  around  them  during 
their  night,  and  behold  all  the  stars  shining,  they  cannot 
imagine  why  the  planets,  which  are  dark  bodies,  should 
shine,  concluding,  that  since  the  sun  does  not  illumine 
themselves,  the  whole  universe  must  be  in  darkness. 

Caroline.  I  confess  that  I  was  one  of  these  ignorant 
people ;  but  I  am  now  very  sensible  of  the  absurdity  of 
such  an  idea.  To  the  inhabitants  of  the  other  planets, 
then,  we  must  appear  as  a  little  star  ? 

Mrs,  JB.  Yes,  to  those  wliich  revolve  round  our  sun  ; 
for  since  those  which  may  belong  to  other  systems  (and 
whose  existence  is  only  hypothetical,)  are  invisible  to  us, 
it  is  probable,  that  we  also  are  invisible  to  them. 

Emihj,  But  they  may  see  our  sun  as  we  do  theirs,  in 
appearance  a  fixed  star  ? 

Mrs,  B,  No  doubt,  if  the  beings  who  inhabit  those 
planets  are  endowed  with  senses  similar  to  ours.  By  the 
same  rule,  we  must  appear  as  a  moon,  to  the  inhabitants 
of  our  moon  ;  but  on  a  larger  scale,  as  the  surface  of  the 
earth  is  about  thirteen  times  as  large  as  that  of  the  moon. 

Emily,  The  moon,  Mrs.  B.,  appears  to  move  in  a 
different  direction,  and  in  a  different  manner  from  the 
stars  1 

Mrs.  B,  I  shall  defer  the  explanation  of  the  motion 
of  the  moon,  till  our  next  interview,  as  it  would  prolong 
our  present  lesson  too  much. 

315.     How  must  the  earth  appear  to  the  inhabitants  of  other 

planets  ^ 3!  6.     How  much  larger  does  the  earth  appear  viewed 

at  the  mooUj  than  the  moon  appenrK-  viewed  at  the  earth  ? 


ON  THE  PLANETS.  89 

CONVERSATION  VII. 

ON  THE  PLANETS. 

Of  the  Satellites  or  Moons ;  Gravity  diminishes  as  the 
Square  of  the  distance ;  Of  the  Solar  System ;  Of  Co- 
mets ;  Constellations,  Signs  of  the  Zodiach ;  Of  Co^ 
pernicus,  Neivton<,  6^c, 

MRS.  B. 

The  planets  are  distinguished  into  primary  and  secon- 
dary. Those  which  revolve  immediately  about  the  sun 
are  called  primary.  Many  of  these  are  attended  in  their 
course  by  smaller  planets,  which  revolve  round  them  : 
these  are  called  secondary  planets,  satellites,  or  moons. 
Such  is  our  moon  which  accompanies  the  earth,  and  is 
carried  with  it  round  the  sun. 

Emily.  How  then  can  you  reconcile  the  motion  of  the 
secondary  planets  to  the  laws  of  gravitation ;  for  the  sun 
is  much  larger  than  any  of  the  primary  planets ;  and  is 
not  the  power  of  gravity  proportional  to  the  quantity  of 
matter  ? 

Caroline,  Perhaps  the  sun,  though  much  larger,  may 
be  less  dense  than  the  planets.  Fire  you  know  is  very 
light,  and  it  may  contain  but  little  matter  though  of  great 
magnitude. 

Mrs,  B,  We  do  not  know  of  what  kind  of  matter  the 
sun  is  made ;  but  we  may  be  certain,  that  since  it  is  the 
general  centre  of  attraction  of  our  system  of  planets,  it 
must  be  the  body  which  contains  the  greatest  quantity  of 
matter  in  that  system. 

You  must  recollect,  that  the  force  of  attraction  is  not 
only  proportional  to  the  quantity  of  matter,  but  to  the 
degree  of  proximity  of  the  attractive  body :  this  power  is 
weakened  by  being  diffused,  and  diminishes  as  the  squares 
of  the  distances  increase.     The  square  is  the  product  of 

317.     How  are  the  planets  distinguished  ? 318.     What  are 

the  primary  planets .? 319.     What  are  the  secondary  planets  ^ 

320.     By  what  other  names  are  the  secondary  planets  called  ? 

321 .     To  what  is  the  force  of  attraction  proportional  besides 

the  quantity  of  matter  in  the  attracting  bodies  ^  ■      322.     What 
is  meant  by  the  square  of  distance  ^ 
8* 


pO  ON  THE  PLANETS* 

a  number  multiplied  by  itself;  so  that  a  planet  situated  at 
twice  the  distance  at  which  we  are  from  the  sun  would 
gravitate  four  times  less  than  we  do ;  for  the  product  of 
two  multiplied  by  itself  is  four. 

Caroline,  Then  the  more  distant  planets  move  sJou  er 
in  their  orbits  ;  for  their  projectile  force  must  be  propor- 
tioned to  that  of  attraction  ?  But  I  do  not  see  how  this 
accounts  for  the  motion  of  the  secondary  round  the  pri- 
mary planets,  in  preference  to  the  sun. 

Emily.  Is  it  not  because  the  vicinity  of  the  primary 
planets  renders  their  attraction  stronger  than  that  of  the 
sun. 

Mrs.  B,  Exactly  so.  But  since  the  attraction  be- 
tween bodies  is  mutual,  the  primary  planets  are  also  at- 
tracted by  the  satellites,  which  revolve  round  them.  The 
moon  attracts  the  earth,  as  well  as  the  earth  the  moon  ; 
but  as  the  latter  is  the  smaller  body,  her  attraction  is  pro- 
portionally less ;  therefore  neither  the  earth  revolves  round 
the  moon,  nor  the  moon  round  the  earth ;  but  they  both 
revolve  round  a  point,  which  is  their  common  centre  of 
gravity,  and  which  is  as  much  nearer  the  earth  than  the 
moon,  as  the  gravity  of  the  former  exceeds  that  of  the 
latter. 

Emily,  Yes,  I  recollect  your  saying,  that  if  two  bodies 
were  fastened  together  by  a  wire  or  bar,  their  common 
centre  of  gravity  would  be  in  the  middle  of  the  bar,  pro- 
vided the  bodies  were  of  equal  weight ;  and  if  they  diflfered 
in  weight,  it  would  be  nearer  the  larger  body.  If  then 
the  earth  and  moon  had  no  projectile  force  which  pre- 
vented their  mutual  attraction  from  bringing  them  to- 
gether, they  would  meet  at  their  common  centre  of  gravity. 

Caroline,  The  earth  then  has  a  great  variety  of  mo- 
tions, it  revolves  round  the  sun,  upon  its  axis,  and  round 
the  point  towards  which  the  moon  attracts  it. 

Mrs.  B.  Just  so ;  and  this  is  the  case  with  every 
planet  which  is  attended  by  satellites.  The  complicated 
effect  of  this  variety  of  motions,  produces  certain  irregu- 
larities, which,  however,  it  is  not  necessary  to  notice  at 
present. 

323.  How  much  less  does  a  planet  gravitate  towards  the  sun 
than  the  earth,  at  twice  the  distance  of  the  earth  from  the  sun  ? 

— 394.  Why  does  not  the  sun  attract  the  secondary  planets 
from  their  primaries  ? 325.  What  motion  has  the  earth  be- 
sides that  about  the  sun  and  on  its  own  axis  ? 326.     Where  is 

the  common  centre  of  gravity  to  the  sun  and  mx)ou  ? 


ON  THE  PLANETS.  91 

The  planets  act  on  the  sun  in  the  same  manner  as  they 
are  themselves  acted  on  by  their  satellites ;  for  attraction, 
you  must  remember,  is  always  mutual ;  but  the  gravity  of 
the  planets  (even  v^^hen  taken  collectively)  is  so  trifling 
compared  with  that  of  the  sun,  that  they  do  not  cause  the 
latter  to  move  so  much  as  one  half  of  his  diameter.  The 
planets  do  not,  therefore,  revolve  round  the  centre  of  the 
sun,  but  round  a  point  at  a  small  distance  from  its  centre, 
about  which  the  sun  also  revolves. 

Emily.  I  thought  the  sun  had  no  motion  1 
Mrs,  B,  You  were  mistaken  ;  for  besides  that  which 
I  have  just  mentioned,  which  is  indeed  very  inconsidera- 
ble, he  revolves  on  his  axis  ;  this  motion  is  ascertained 
by  observing  certain  spots  which  disappear,  and  re-appear 
regularly  at  stated  times.* 


*  The  sun  is  a  spherical  body,  situated  near  the  centre  of  gravi- 
ty in  the  system  of  planets,  of  which  our  earth  is  one.  Its  dia- 
meter is  077,547  Enghsh  miles  ;  or  equal  to  100  diameters  of  the 
earth  ;  and  therefore  its  cubick  magnitude  must  exceed  that  of  the 
earth  one  million  of  times.  It  revolves  round  its  axis  in  25  days, 
and  10  hours,  which  has  been  determined  by  means  of  several  dark 
spots  seen  with  telescopes  on  that  luminary.  Dr.  ITerschel  sup- 
poses these  spots  in  the  sun  to  be  the  appearancb  of  the  opaque 
body  of  the  sun  through  the  openings  in  his  luminous  atmosphere. 

Its  similarity  to  the  other  globes  of  the  solar  system,  in  solidity, 
atmosphere,  surface  diversified  with  mountains  and  valleys,  and 
rotation  on  its  axis,  lead  us  to  suppose,  that  it  is  most  probably  in- 
habited like  the  rest  of  the  planets,  by  beings  whose  organs  are 
adapted  to  their  peculiar  circumstances. 

Though  it  may  be  objected,  from  the  effects  produced  at  the 
distance  of  95,000,000  miles,  that  every  thing  must  be  scorched  up 
at  its  surface,  yet  many  facts  show  that  heat  is  produced  by  the 
sun's  rays  only  when  they  act  on  a  suitable  medium  ;  or  when 
radiated  and  reflected  by  suitable  surfaces.  On  the  tops  of  moun- 
tains of  sufficient  height,  we  always  find  regions  of  ice  and  snow  ; 
though  if  the  solar  rays  themselves  conveyed  all  the  heat  we  find 
on  this  globe,  it  ought  tol9e  hottest  where  their  course  is  the  least 
interrupted. 


327.  Do  the  planets  revolve  round  the  centre  of  the  sun  ? 

328.  Around  what  do  they  revolve  ? 329.     Has  the  sun  any 

motion  ?- 330.     How  is  it  known  that  the  sun  turns  on  its  axis  ? 

331.     How  much  greater  is  the  diameter  of  the  sun  than  of 

the  earth  ? 332.     How  much  does  his  cubick  magnitude  exceed 

that  of  the   earth  9 333.      What    does  Dr.  Herschel    suppose 

the  dark  spots  on  the  sun's  disk  to  be  ? 334.   What  are  we  led  to 

suppose  from  the  similarity  of  the  sun  to  the  other  globes  of  the 
solar  system  9 


92  ON  THE  PLANETS. 

Caroline.  A  planet  has  frequently  been  pointed  out  to 
me  in  the  heavens  ;  but  1  could  not  perceive  that  its  mo- 
tion ditlered  from  that  of  the  fixed  stars,  which  only  appear 
to  move. 

Mi's,  B,  The  great  distance  of  the  planets  renders 
their  motion  apparently  so  slow,  that  the  eye  is  not  sen- 
sible of  their  progress  in  their  orbit,  unless  we  watch  them 
for  some  considerable  length  of  time  :  in  different  seasons 
they  appear  in  different  parts  of  the  heavens.  The  most 
accurate  idea  I  can  give  you  of  the  situation  and  motion 
of  the  planets,  will  be  by  the  examination  of  this  diagram, 
(plate  VII.  tig.  1.)  representing  the  solar  system,  in  which 
you  will  find  every  planet  with  its  orbit  delineated. 

Emily,  But  the  orbits  here  are  all  circular,  and  you  said 
that  they  were  elliptical.  The  planets  appear  too,  to  be 
moving  round  the  centre  of  the  sun  ;  whilst  you  told  us  that 
they  moved  round  a  point  at  a  little  distance  from  thence. 

Mrs,  IB,  The  orbits  of  the  planets  are  so  nearly  cir- 
cular, and  the  common  centre  of  gravity  of  the  solar  sys- 
tem so  near  the  centre  of  the  sun,  that  these  deviations 
are  scarcely  worth  observing.  The  dimensions  of  the 
planets,  in  their  true  proportions,  you  will  find  delineated 
in  fig.  2. 

Mercury  is  the  planet  nearest  the  sun  ;  his  orbit  is  con- 
sequently contained  within  ours ;  but  his  vicinity  to  the 
sun  occasions  his  being  nearly  lost  in  the  brilliancy  of 
his  rays;  and  when  we  see  the  sun,  he  is  so  dazzling 
that  very  accurate  observations  cannot  be  made  upon 
Mercury.  He  performs  his  revolution  round  the  sun  in 
about  87  days,  which  is  consequently  the  length  of  his 
year.  The  time  of  his  rotation  on  his  axis  is  not  known  ; 
his  distance  from  the  sun  is  computed  to  be  37  millions 
of  miles,  and  his  diameter  3180  miles.  The  heat  of  this 
planet  is  so  great,  that  water  cannot  exist  there,  but  in  a 
state  of  vapour,  and  metals  would  be  liquefied.* 

*  The  intenseness  of  the  sun's  heat,  which  is  in  the  same  pro- 
portion as  his  light,  is  seven  times  as  great  in  Mercury  as  with  us ; 

335.     Can  the  motion  of  the  planets  be  seen  by  the  naAed  eye  ? 

336.     What   is   the   design   of  Fig.    1,  plate  VTl  ?— 337. 

Which  figure  exhibits  the  dimensions  of  the  planets  m  their  true 

proportions  ? 338.     What  planet  is  nearest  the  sun  ? 339. 

In  what  time  does  Mercury  revolve  round  the  sun  ? — —340.    What 

is  his  distance  from  the  sun  ? 341.     What  is  his  diameter  ? 

342.  How  does  the  intenseness  of  the  sun's  heat  at  Mercury  com- 
pare with  it  at  our  earth  ? 


CTN  THE  PLANET«.  93 

Caroline,     Oh,  what  a  dreadful  climate. 

Mrs,  B.  Though  we  could  not  live  there,  it  may  be 
perfectly  adapted  to  other  beings  destined  to  inhabit  it. 

Venus,  the  next  in  the  order  of  planets,  is  68  millions 
of  miles  from  the  sun ;  she  revolves  about  her  axis  in  23 
hours  and  21  minutes,  and  goes  round  the  sun  in  244 
days  17  hours.  The  orbit  of  Venus  is  also  v/ithin  ours  ; 
during  one  half  of  her  course  in  it,  we  see  her  before  sun- 
rise, and  she  is  called  the  morning  star  ;  in  the  other  part 
of  her  orbit,  she  rises  later  tha,n  the  sun.* 

Caroline.  In  that  case,  we  cannot  see  her,  for  she 
must  rise  in  the  day  time  ? 

Mrs,  B,  True ;  but  when  she  rises  later  than  the  sun, 
she  also  sets  later ;  so  that  we  perceive  her  approaching 
the  horizon  after  sun-set :  she  is  then  called  Hesperus,  or 
the  evening  star.  Do  you  recollect  those  beautiful  lines 
of  Milton  ? 

Now  came  still  evening  on,  and  twilight  gray 
Had  in  her  sober  livery  all  things  clad  : 
Silence  accompanied  ;  for  beast  and  bird, 
They  to  their  grassy  couch,  these  to  their  nests 
Were  slunk,  all  but  the  wakeful  nightingale  ; 
She  all  night  long  her  amorous  descant  sung ; 
Silence  was  pleas'd  ;  now  glow'd  the  firmament 
With  living  sapphires,     Hesperus,  that  led 
The  starry  host,  rode  brightest,  till  the  moon 
Rising  in  clouded  majesty,  at  length 
Apparent  queen  unveil'd  her  peerless  light, 
And  o'er  the  dark  her  silver  mantle  threw. 


so  that  water  there  would  be  carried  off  in  the  shape  of  steam,  for 
by  experiments  with  the  thermometer,  it  appears  that  a  heat  seven 
times  greater  than  that  of  the  sun's  beams  in  summer  will  serve  to 
make  water  boil. 

*  In  most  treatises  on  Astronomy,  Mercury  and  Venus  are  call- 
ed inferiour,  and  those  more  distant  from  the  sun  than  our  earth, 
superiour  planets  ;  but,  it  is  considered  a  more  proper  distinction, 
to  call  the  former  interiour  and  the  latter  exteriour  planets. 


343.     How  much  greater  heat  is  required  to  make  water  hoil, 

tlian   that  of  the  sun  in  summer  at  the  earth  f 344.     How  far 

is  Venus  from  the  sun  ? 345.     In  what  time  does  it  revolve 

round  the  sun  ? 346.     In  what  time  does   it  revolve  upon  its 

axis  ? 347.     When  is  Venus  called  the  morning  and  when  the 

evening  star  ? 348.     By  lohat  name  have  Mercury  and  Venus 

usually  been  distinguished  from  the  other  planets  ? 349.     How 

should  the  planets  more  distant,  and  those  less  distant  from  the 
sun  than  the  earth,  be  distinguished  from  each  other  ? 


94  ON  THE  PLANETS. 

The  planet  next  to  Venus  is  the  Earth,  of  which  we 
shall  soon  speak  at  full  length.  At  present  I  shall  only 
observe,  that  we  are  95  millions  of  miles  distant  from  the 
sun,  that  we  perform  our  annual  revolution  in  365  days, 
5  hours,  and  49  minutes  ;  and  are  attended  in  our  course 
by  a  single  moon. 

Next  follows  Mars.  He  can  never  come  between  us 
and  the  sun,  like  Mercury  and  Venus  ;  his  motion  is, 
however,  very  perceptible,  as  he  may  be  traced  to  differ- 
ent situations  in  the  heavens  ;  his  distance  from  the  sun 
is  144  millions  of  miles  ;  he  turns  round  his  axis  in  24 
hours  and  39  minutes  ;  and  he  performs  his  annual  revo- 
lution in  about  687  of  our  days :  his  diameter  is  4120 
miles.  Then  follow  four  very  small  planets,  Juno,  Ce- 
res, Pallas,  and  Vesta,  which  have  been  recently  disco- 
vered, but  whose  dimensions  and  distances  from  the  sun 
have  not  been  very  accurately  ascertained.* 

Jupiter  is  next  in  order  :  this  is  the  largest  of  all  the 
planets.  He  is  about  490  millions  of  miles  from  the  sun, 
and  completes  his  annual  period  in  nearjy  12  of  our  years. 
He  turns  round  his  axis  in  about  ten  hours.  He  is  above 
1200  times  as  big  as  our  earth  ;  his  diameter  being  86,000 
miles.  The  respective  proportions  of  the  planets  cannot, 
therefore,  you  see,  be  conveniently  delineated  in  a  dia- 
gram.    He  is  attended  by  four  moons.t 

*  These  anomalous  bodies,  so  unlike  the  other  primary  planets, 
Dr.  Herschel  has  denominated  Asteroids.  Probably  they  are  the 
fragments  of  some  planet ;  or  perhaps  other  similar  bodies  abound 
in  the  solar  system,  though  they  have  hitherto,  from  their  small- 
ness  or  darkness,  escaped  observation. 

t  Jupiter  is  surrounded  by  cloudy  substances,  subject  to  fre- 
quent changes  in  their  situation  and  appearance,  called  Belts. 
These  Belts  are  sometimes  of  a  regular  form  ;  sometimes  inter- 
rupted and  broken  ;  and  sometimes  not  at  all  to  be  seen. 


350.     How  far  distant  from  the  sun  is  the  earth  ? 351.     In 

what  time  does  it  revolve  round  the  sun  r 352.     Which  planet 

is  next  to  the  earth  in  distance  from  the  sun. 353.     How  far 

is  Mars  from  the  sun  ? 354.     How  long  time  is  occupied  in  his 

revolution  about  the  sun  ? 355.     What  four  small  planets  are 

next  to  Mars  in  distance  from  the  sun  ? 356.     What  did  Dr. 

Herschel  call  these  planets? -357.     What  is  the  distance  of 

Jupiter  from  the  sun  .•' 358.     In  what  time  does  Jupiter  com- 
plete his  revolution  ? 359.     How  much  larg-er  is  Jupiter  than 

our  earth  ? 360.      How  many  satellites  has  this  planet  ?— ^ 

3151.     By  ichat  is  Jupiter  s^t^rounded  9 


ON  THE  PLANETS.  95 

The  next  planet  is  Saturn,  whose  distance  from  the  sun 
is  about  900  millions  of  miles  ;  his  diurnal  rotation  is  per- 
formed in  10  hours  and  a  quarter  : — his  annual  revolution 
in  nearly  30  of  our  years.  His  diameter  is  79,000  miles. 
This  planet  is  surrounded  by  a  luminous  ring,  the  nature 
of  which,  astronomers  are  much  at  a  loss  to  conjecture  ; 
he  has  seven  moons.*  Lastly,  we  observe  the  Georgium 
3idus,  discovered  by  Dr.  Herschel,  and  which  is  attended 
by  six  moons. 

Caroline.  How  charming  it  must  be  in  the  distant 
planets,  to  see  several  moons  shining  at  the  same  time  ; 
I  think  I  should  like  to  be  an  inhabitant  of  Jupiter  or 
Saturn. 

Mrs,  B,  Not  long,  I  believe.  Consider  what  ex- 
treme cold  must  prevail  in  a  planet,  situated  as  Saturn  is, 
at  nearly  ten  times  the  distance  at  which  we  are  from  the 
sun.  Then  his  numerous  moons  are  far  from  making  so 
splendid  an  appearance  as  ours  ;  for  they  can  reflect  only 
the  light  which  they  receive  from  the  sun  ;  and  both  light 
and  heat  decrease  in  the  same  ratio  or  proportion  to  the 
distances  as  gravity.  Can  you  tell  me  now  how  much 
more  light  we  enjoy  than  Saturn  1 

Caroline,  The  square  of  ten,  is  a  hundred  ;  therefore 
Saturn  has  a  hundred  times  less — or  to  answer  your  ques- 
tion exactly,  we  have  a  hundred  times  more  light  and  heat 
than  Saturn — this  certainly  does  not  increase  my  wish 
to  become  one  of  the  poor  wretches  who  inhabit  that 
planet.t 

*  This  ring  is  set  edgewise  round  it,  and  the  distance  of  the  ring- 
from  the  planet  is  equal  to  the  breadth  of  the  ring.  The  sun  shines 
for  almost  fifteen  of  our  years  together  on  the  northern  side  of  the 
ring  ;  then  goes  off,  and  shines  as  long  on  the  southern  side  of  it, 
so  there  is  but  one  day  and  one  night  on  each  side  of  the  ring,  in 
the  time  of  Saturn's  whole  revolution  about  the  sun,  which  takes 
up  almost  thirty  of  our  years. 

t  The  sun's  light  at  Saturn  is  1000  times  as  great  as  the  light  of 
the  full  moon  is  to  us. 

362.     What  planet  is  next  in  order  as  to  distance  from  the  sun  ^ 

-^ .363.     What  is  its  distance  from  the  sun  ? 364.     In  what 

time  does  it  revolve  round  that  luminary  ? 365.     What  is  its 

diameter  } 366.     How  many  moons  has  Saturn  .? 367.     By 

what  is  this  planet  surrounded? 368.      What  is   said  in   the, 

note  of  Saturn's  ring  9 369.     How  many  moons  has  Herschel 

or  the  Georgium  Sidus? 370.     How  much  more  light  and  heat 

do  we   enj)y  thnn  Saturn.' 371.     How  much  greater  is  ths 

sun's  light  Hi  Saturn  than  the  moon's  light  at  the  earth  ? 


96  ON  THE  PLANETi5. 

Mrs,  B,  May  not  the  inhabitants  of  Mercury,  with 
equal  plausibility,  pity  us,  for  the  insupportable  coldness 
of  our  situation,  and  those  of  Jupiter  and  Saturn  for  our 
intolerable  heat  ?  The  Almighty  Power  which  created 
these  planets,  and  placed  them  in  their  several  orbits,  has 
no  donbt  peopled  them  with  beings  whose  bodies  are 
adapted  to  the  various  temperatures  and  elements  in 
which  they  are  situated.  If  we  judge  from  the  analogy 
of  our  own  earth,  or  from  that  of  the  great  and  universal 
beneficence  of  Providence,  we  must  conclude  this  to  be 
the  case. 

Caroline,     Are  not  comets  also  supposed  to  be  planets  ? 

Mrs,  B,  Yes,  they  are  ;  for  by  the  re-appearance  of 
some  of  them,  at  stated  times,  they  are  known  to  revolve 
round  the  sun,  but  in  orbits  so  extremely  eccentrick,  that 
they  disappear  for  a  great  number  of  years.  If  they  are 
inhabited,  it  must  be  by  a  species  of  beings  very  different, 
not  only  from  the  inhabitants  of  this,  but  from  those  of  any 
of  the  other  planets,  as  they  must  experience  the  greatest 
vicissitudes  of  heat  and  cold  ;  one  part  of  their  orbit  being 
so  near  the  sun,  that  their  heat,  when  there,  is  computed 
to  be  greater  than  that  of  red-hot  iron ;  in  this  part  of  its 
orbit,  the  comet  emits  a  luminous  vapour,  called  the  tail, 
which  it  gradually  loses  as  it  recedes  from  the  sun ;  and 
the  comet  itself  totally  disappears  from  our  sight,  in  the 
more  distant  parts  oif  its  orbit,  which  extends  considerably 
beyond  that  of  the  furthest  planet. 

The  number  of  comets  belonging  to  our  system  cannot 
be  ascertained,  as  some  of  them  are  whole  centuries  be- 
fore they  make  their  re-appearance.  The  numbers  that 
are  known  by  their  regular  re-appearance  is  only  three.* 

Emily.     Pray,  Mrs.  B.  what  are  the  constellations  ? 


^  Above  500  comets  have  appeared  since  the  commencement  of 
the  Christian  era  ;  and  accounts  of  many  mor^  are  extant. 

,/ 

, _ — . — . _ -i 

372.     What  are  the  comets   supposed  to  be  ? 373.     From 

what  fact  is  it  concluded  that  the  comets  are  planets  ? 374. 

Whv  must  the  inhabitant*  of  comets,  if  they  are  inhabited,  expe- 
rience great  vicissitudes  of  heat  and  cold/ 375.     When  in  that 

part  of  their  orbit  nearest  the  sun,  whptls  their  heat  computed  to 
be  ? ^76.  How  many  comets  are  known  by  their  regular  re- 
appearance ^ 377.     Horn  many  different  ones  have  been  noticed 

^ince  the  commencement  of  the  Christian  era  f 


OS  tHE  PLANETS.  9*5^ 

3Irs.  IB.  They  are  the  fixed  stars,  which  the  ancients, 
in  order  to  recognise  them,  formed  into  groupes,  and 
gave  the  names  of  the  figures,  which  you  find  delineated 
on  tiie  celestial  globe.  In  order  to  show  their  proper  situ- 
ations in  the  heavens,  they  should  be  painted  on  the  in- 
ternal surface  of  a  hollow  sphere,  from  the  centre  of  which 
you  should  view  them  ;  you  would  then  behold  them,  as 
they  appear  to  be  situated  in  the  heavens.  The  twelve  con- 
stellations, called  the  signs  of  the  zodiack,  are  those  which 
are  so  situated,  that  the  earth  in  its  annual  revolution  passes 
directly  between  them  and  the  sun.  Their  names  are 
Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Libra,  Scor- 
pio, Sagittarius,  Capricornus,  Aquarius,  Pisces ;  the 
whole  occupying  a  complete  circle,  or  broad  belt,  in  the 
heavens,  called  the  zodiack.  (plate  VIIL  fig.  1.)  Hence, 
a  right  line  drawn  from  the  earth,  and  passing  through 
the  sun,  would  reach  one  of  these  constellations,  and  the 
sun  is  said  to  be  in  that  constellation  at  which  the  line 
terminates  :  thus,  when  the  earth  is  at  A,  the  sun  would 
appear  to  be  in  the  constellation  or  sign  Aries  ;  when  the 
earth  is  at  B,  the  sun  would  appear  in  Cancer;  when 
the  earth  was  at  C,  the  sun  would  be  in  Libra ;  and  when 
the  earth  was  at  D,  the  sun  would  be  in  Capricorn.  This 
circle,  in  which  the  sun  thus  appears  to  move,  and  which 
passes  through  the  middle  of  the  zodiack,  is  called  the 
ecliptick. 

Caroline.  But  many  of  the  stars  in  these  constella- 
tions appear  beyond  the  zodiack. 

Mrs.  B.  We  have  no  means  of  ascertaining  the  dis- 
tance of  the  fixed  stars.  When,  therefore,  they  are  said 
to  be  in  the  zodiack,  it  is  merely  implied,  that  they  are 
situated  in  that  direction,  and  that  they  shine  upon  us 
through  that  portion  of  the  heavens,  which  we  call  the 
zodiack.* 


*  An  easy  distinction  between  a  planet  and  a  fixed  star  is  this — 

378.     What  are  the  constellations  ? 379.     In  what   manner 

can  we  have  an  idea  of  their  proper  situations  ? 380.     What 

are  the  names  of  the  twelve  constellations  ? 381 .   What  is  meant 

when  the  sun  is  said  to  be  in  a  particular  constellation  ? 382- 

How  would  you  illustrate  this  by  the  figure  ^ 383.  What  is  the 

circle  called  in  which  the  sun  appears  to  move  through  the  zo- 
diack ^ 384.     What  is  to  be  understood  by  the  signs  or  cod* 

stellations  being  in  the  zodiack.' 385.     How  may  a  fixed  siar 

be  easily  distinguished  from  a  planet  ? 

9 


98  ON  THE  PLANETS. 

Emihj,  But  are  not  those  large  bright  stars,  which  are 
called  stars  of  the  first  magnitude,  nearer  to  us,  than 
those  small  ones  which  we  can  scarcely  discern  ? 

Mrs.  B.  It  may  be  so  ;  or  the  difference  of  size  and 
brilliancy  of  the  stars  may  proceed  from  their  difference 
of  dimensions  ;  this  is  a  point  which  astronomers  are  not 
enabled  to  determine.  Considering  them  as  suns,  I  see 
no  reason  why  different  suns  should  not  vary  in  dimen- 
sions as  well  as  the  planets  belonging  to  them.* 

Etnily,  What  a  wonderful  and  beautiful  system  this 
is,  and  how  astonishing  to  think  that  every  fixed  star  may 
probably  be  attended  by  a  similar  train  of  planets  ! 

Caroline,  You  will  accuse  me  of  being  very  incredu- 
lous, but  I  cannot  help  still  entertaining  some  doubts,  and 
fearing  that  there  is  more  beauty  than  truth  in  this  system. 
It  certainly  may  be  so  ;  but  there  does  not  appear  to 
me  to  be  sufficient  evidence  to  prove  it.  It  seems  so 
plain  and  obvious  that  the  earth  is  motionless,  and  that 
the  sun  and  stars  revolve  round  it  ;  your  solar  system, 
you  must  allow,  is  directly  in  opposition  to  the  evidence 
of  our  senses. 

Mrs.  B.  Our  senses  so  often  mislead  us,  that  we 
should  not  place  implicit  reliance  upon  them. 

Caroline.  On  what  then  can  we  rely,  for  do  we  not 
receive  all  our  ideas  through  the  medium  of  our  senses  ? 

Mrs.  B.  It  is  true  that  they  are  our  primary  source  of 
knowledge  ;  but  the  mind  has  the  power  of  reflecting, 
judging,  and  deciding  upon  the  ideas  received  by  the  or- 
gans of  sense.  This  faculty,  which  we  call  reason,  has 
frequently  proved  to  us,  that  our  senses  are  liable  to  err. 


the  former  shines  with  a  steady  light,  but  the  latter  is  constantly 
twinkling.  What  it  is  which  occasions  this  twinkling  or  scintilla- 
tion of  a  star,  yet  remains  undecided. 

*  To  the  bare  eye  the  stars  appear  of  some  sensible  magnitude, 
owing  to  the  glare  of  light  arising  from  the  numberless  reflections 
of  the  rays  in  coming  to  the  eye  ;  this  leads  us  to  imagine  that  the 
stars  are  much  larger  than  they  would  appear,  if  we  saw  them  only 
b}^  the  few  rays  which  come  directly  from  them,  so  as  to  enter  the 
eye,  without  being  intermixed  with  others. 


386.     On  what  is  the  different  size  and  brilliancy  of  the  fixed 

stajrs  depending  ? 387.     What  caitses  the  fixed  stars  to  appear 

to  UB  larger  than  they  should  appear  f 


ON  THE  PLANETS. 


99 


If  yoa  have  ever  sailed  on  the  water,  with  a  very  steady 
breeze,  you  must  have  seen  the  houses,  trees,  and  every 
object  move,  while  vou  were  sailing. 

Caroline.  I  remember  thinking  so,  when  I  was  very 
young  ;  but  I  now  know  that  their  motion  is  only  appa- 
rent. It  is  true  that  my  reason,  in  this  case,  corrects  the 
errour  of  my  sight. 

Mrs.  B. '  It  teaches  you  that  the  apparent  motion  ot 
the  objects  on  shore,  proceeds  from  your  being  yourself 
movinfif,  and  that  you  are  not  sensible  of  your  own  motion 
because  you  meet  with  no  rp?istance.  It  is  only  when 
some  obstacle  impedes  our  motion,  that  we  are  conscious 
of  moving  ;  and  if  you  were  to  close  your  eyes  when  you 
were  sailing  on  calm  water,  with  a  steady  wind,  you 
would  not  perceive  that  you  moved,  for  you  could  not  ieei 
it,  and  you  could  see  it  only  by  observing  the  change  oi 
place  of  the  objects  on  shore.  So  it  is  with  the  motion  of 
the  earth  ;  every  thing  on  it>  surface,  and  the  air  that 
surrounds  it,  accomoanies  it  in  its  revolution  ;  it  meets 
^ilh  no  resistance :  thereiore,  like  the  crew  ot  a  vessel 
sailing  with  a  fair  wind,  in  a  calm  sea,  we  are  insensible 
of  our  motion. 

Caroline.  But  the  principal  reason  why  the  crew  of  a 
vessel  in  a  calm  sea  do  not  perceive  their  motion,  is,  be- 
cause they  move  exceedingly  slowly  :  while  the  earth, 
you  say,  revolves  with  great  velocity. 

Mrs.  B.  It  is  not  because  they  move  slowly,  but  be- 
■cause  they  move  steadily,  and  meet  with  no  irregular  re- 
sistances, that  the  crew  of  a  vessel  do  not  perceive  their 
motion  ;  for  they  would  be  equally  insensible  to  it,  with 
the  strongest  wind,  provided  it  were  steady,  that  they 
mailed  with  it,  and  that  it  did  not  agitate  the  water  ;  but 
this  last  condition,  you  know,  is  not  possible,  for  the  wind 
will  always  produce  waves  which  offer  more  or  less  resist- 
ance to  the  vessel,  and  then  the  motion  becomes  sensible, 
because  it  is  unequal. 

Caroline.  But,  granting  this,  the  crew  of  a  vessel  have 
a  proof  of  their  motion,  though  insensible,  which  the  in- 
habitants of  the  earth  cannot  have, — the  apparent  motion 
of  the  objects  on  shore. 

388.     What  familiar  illustration  is  given  to  show  why  we    do 

not  perceive  the  motion  of  the  earth  in  its  revolutions  ? 389. 

Why  do  we  not  perceive  its  motion  t 


100  ON  THE  PLANETS. 

3Irs,  B.  Have  we  not  a  similar  proof  of  the  earth's 
motion,  in  the  apparent  motion  of  the  sun  and  stars  ?  Ima- 
gine the  earth  to  be  sailing  round  its  axis,  and  succes- 
sively passing  by  every  star,  which,  like  the  objects  on 
land,  we  suppose  to  be  moving  instead  of  ourselves.  I 
have  heard  it  observed  by  an  aerial  traveller  in  a  balloon, 
that  the  earth  appears  to  sink  beneath  the  balloon,  in- 
stead of  the  balloon  rising  above  the  earth. 

It  is  a  law  which  we  discover  throughout  nature,  and 
worthy  of  its  great  Author,  that  all  its  purposes  are  ac- 
complished by  the  most  simple  means  ;  and  what  reason 
have  we  to  suppose  this  law  infringed,  in  order  that  we 
may  remain  at  rest,  while  the  sun  and  stars  move  round 
us  ;  their  regular  motions,  which  are  explained  by  the 
laws  of  attraction  on  the  first  supposition,  would  be  un- 
intelligible on  the  last,  and  the  order  and  harmony  of  the 
universe  be  destroyed.  Think  what  an  immense  circuit 
the  sun  and  stars  would  make  daily,  were  their  apparent 
motions  real.  We  know  many  of  them  to  be  bodies 
more  considerable  than  our  earth  ;  for  our  eyes  vainly 
endeavour  to  persuade  us,  that  they  are  little  brilliants 
sparkling  in  the  heavens,  while  science  teaches  us  that 
they  are  immense  spheres,  whose  apparent  dimensions 
are  diminished  by  distance.  Why  then  should  these 
enormous  globes  daily  traverse  such  a  prodigious  space, 
merely  to  prevent  the  necessity  of  our  earth's  revolving 
on  its  axis  ? 

Caroline.  I  think  I  must  now  be  convinced.  But 
you  will,  I  hope,  allow  me  a  little  time  to  familiarize  my- 
self to  an  idea  so  different  from  that  which  I  have  been 
accustomed  to  entertain.  And  pray,  at  what  rate  do  wc 
move  ? 

3Irs,  B,  The  motion  produced  by  the  revolution  of 
the  earth  on  its  axis,  is  about  eleven  miles  a  minute,  to  an 
inhabitant  of  London. 

Emily,  But  does  not  every  part  of  the  earth  move 
with  the  same  velocity  ? 

390.     In  case  the  earth  revolves  every  24  hours,  do  not  the  sun 

and  stars  appear  to  us  as  if  they  revolved  about  the  earth  ? 391. 

What  law  is  mentioned  that  we  discover  throughout  nature  ? 

392.     Why  does  this  law  make  it  more  probable  that  the  earth  re 

volves  than  that  the  sun  and  stars  do  ? 303.     How  fast  doc:* 

z,  person  move  in  the  latitude  of  London,  in  consequence  of  (he 
earth's  motion  upon  its  axis  ? 


O^  THE  PLANETS.  101 

3Irs,  J5.  A  moment's  reflection  would  convince  you 
of  the  contrary  ;  a  person  at  the  equator  must  move 
quicker  than  one  situated  near  the  poles,  since  they  both 
perform  a  revolution  in  24  hours. 

Emily,  True,  the  equator  is  furthest  from  the  axis  of 
motion.  But  in  the  earth's  revolution  round  the  sun, 
every  part  must  move  with  equal  velocity  ? 

Mrs,  IB,     Yes,  about  a  thousand  miles  a  minute. 

Caroline,  How  astonishing  ! — and  that  it  should  be 
possible  for  us  to  be  insensible  of  such  a  rapid  motion. 
You  would  not  tell  me  this  sooner,  Mrs.  B.,  for  fear  of 
increasing  my  incredulity. 

Before  the  time  of  Newton,  was  not  the  earth  supposed 
to  be  in  the  centre  of  the  system,  and  the  sun,  moon,  and 
stars  to  revolve  round  it  ? 

Mrs,  B,  This  was  the  system  of  Ptolem.y  in  ancient 
times ;  but  as  long  ago  as  the  beginning  of  the  sixteenth 
century  it  was  discarded,  and  the  solar  system,  such  as 
I  have  shown  you,  was  established  by  the  celebrated  as- 
tronomer Copernicus,  and  is  hence  called  the  Copernican 
system.  But  the  theory  of  gravitation,  the  source  from 
which  this  beautiful  and  harmonious  arrangement  flows, 
we  owe  to  the  powerful  genius  of  Newton,  who  lived  at  a 
much  later  period. 

Emily,  It  appears,  indeed,  far  less  difiicult  to  trace 
by  observation  the  motion  of  the  planets,  than  to  divine 
by  what  power  they  are  impelled  and  guided.  I  wonder 
how  the  idea  of  gravitation  could  first  have  occurred  to 
Sir  Isaac  Newton  ? 

Mrs.  B,  It  is  said  to  have  been  occasioned  by  a  cir- 
cumstance from  which  one  should  little  have  expected  so 
grand  a  theory  to  have  arisen. 

During  the  prevalence  of  the  plague  in  the  year  1665, 
Newton  retired  into  the  country  to  avoid  the  contagion  : 
when  sitting  one  day  in  his  orchard  he  observed  an  apple 
fall  from  a  tree,  and  was  led  to  consider  what  could  be 
the  cause  which  brought  it  to  the  ground. 

394.  How  fast  does  the  earth  move  in  its  revolution  about  the 
sun  ? 395.  What  was  the  system  of  Ptolemy  concerning  as- 
tronomy ? 396.     What  is  the    present    system   of  astronomy 

called  ? 397.     When  was  the  Copernican  system  of  astronomy 

adopted  ? 398.     What  important  discovery  did  Newton  make 

touching  the  Copernican  system  ? 399.    What  led  Newton  to 

make  his  discoveries  ? 

9* 


102  ON  THE  EARTH. 

Caroline*  If  I  dared  to  confess  it,  Mrs.  B.,  I  should 
say  that  such  an  inquiry  indicated  rather  a  deficiency 
than  a  superiority  of  intellect.  I  do  not  understand  how 
any  one  can  wonder  at  what  is  so  natural  and  so  common. 

Mrs,  B.  It  is  the  mark  of  superiour  genius  to  find 
matter  for  wonder,  observation,  and  research,  in  circum- 
stances which,  to  the  ordinary  mind,  appear  trivial,  be- 
cause they  are  common,  and  with  which  they  are  satis- 
fied, because  they  arc  natural,  without  reflecting  that  na- 
ture is  our  grand  field  of  observation,  that  within  it  is  con- 
tained our  whole  store  of  knowledge;  in  a  word,  that  to 
study  the  works  of  nature,  is  to  learn  to  appreciate  and 
admire  the  wisdom  of  God.  Thus,  it  was  the  simple  cir- 
cumstance of  the  fall  of  an  apple,  which  led  to  the  discovery 
of  the  laws  upon  which  the  Copernican  system  is  found- 
ed ;  and  whatever  credit  this  system  had  obtained  before, 
it  now  rests  upon  a  basis  from  which  it  cannot  be  shaken. 

Emily.  This  was  a  most  fortunate  apple,  and  more 
worthy  to  be  commemorated  than  all  those  that  have  been 
3ung  by  the  poets.  The  apple  of  discord  for  which  the 
goddesses  contended  ;  the  golden  apples  by  which  Ata- 
lanta  won  the  race ;  nay,  even  the  apple  which  William 
Tell  shot  from  the  head  of  his  son,  cannot  be  compared 
to  this  ! 


CONVERSATION  VIII. 


ON  THE  EARTH. 


Of  the  Terrestrial  Glohe  ;  Of  the  Figttre  of  the  Earth  ; 
Of  the  Pendulum;  Of  the  Variation  of  the  Seasons, 
and  of  the  Length  of  Days  and  Nights ;  Of  the  Causes 
of  the  Heat  of  Summer ;  Of  Solar,  Sidereal^  and  Equal 
or  Mean    Time. 


MRS.  B. 


As  the  earth  is  the  planet  in  which  we  are  the  most 
particularly  interested,  it  is  my  intention  this  morning, 
to  explain  to  yoa  the  effects  resulting  from  its  annual  and 

400.  What  does  Mrs.  Bryan  consider  a  mark  of  superiour  genius  ? 


ON  THE  EARTH.  lOS 

diurnal  motions  ;  but  for  this  purpose  it  will  be  necessa- 
ry to  make  you  acquainted  with  the  terrestrial  globe  :  you 
have  not  either  of  you,  I  conclude,  learnt  the  use  of  the 
globes  ?* 

Caroline,  No ;  I  once  indeed  learnt  by  heart  the 
names  of  the  lines  marked  on  the  globe,  but  as  I  was  in- 
formed they  were  only  imaginary  divisions,  they  did  not 
appear  to  me  worthy  of  much  attention,  and  were  soon 
forgotten. 

Mrs,  B,  You  suppose,  then,  that  astronomers  had 
been  at  the  trouble  of  inventing  a  number  of  lines  to  little 
purpose.  It  will  be  impossible  for  me  to  explain  to  you 
the  particular  effects  of  the  earth's  motion  without  your 
having  acquired  a  knowledge  of  these  lines  :  in  plate 
VIII.  fig.  2.  you  will  find  them  all  delineated  ;  and  you 
must  learn  them  perfectly  if  you  wish  to  make  any  profi- 
ciency in  astronomy. 

Caroline,  I  was  taught  them  at  so  early  an  age  that  I 
could  not  understand  their  meaning  ;  and  I  have  often 
heard  you  say  that  the  only  use  of  words  was  to  convey 
ideas. 

Mrs.  B,  The  names  of  these  lines  would  have  con- 
veyed ideas  of  the  figures  they  were  designed  to  express, 
though  the  use  of  these  figures  might  at  that  time  have  been 
too  difficult  for  you  to  understand.  Childhood  is  the  sea- 
son when  impressions  on  the  memory  are  most  strongly  and 
most  easily  made  :  it  is  the  period  at  which  a  large  stock  of 
ideas  should  be  treasured  up,  the  application  of  which  we 
may  learn  when  the  understanding  is  more  developed. 
It  is,  I  think,  a  very  mistaken  notion  that  children  should 
be  taught  such  things  only,  as  they  can  perfectly  under- 


*  The  earth  is  of  a  globular  form.  For,  1.  The  shadow  of  the 
earth  projected  on  the  moon  in  an  eclipse  is  "always  circular;, 
which  appearance  could  only  be  produced  by  a  spherical  body. 
2.  The  convexity  of  the  surface  of  the  sea  is  evident ;  the  mast  of 
an  approaching  ship  being  seen  before  its  hull.  3.  The  north  pole 
becomes  more  elevated  by  travelling  northward,  in  proportion  to 
the  space  passed  over.  4.  Navigators  have  sailed  round  the  earth, 
and  by  sieering  their  course  continually  westward  arrived,  at 
length,  at  the  place  from  whence  they  departed. 

401.  How  is  it  proved  that  the  earth  is  globular  9^"— -402. 
What  is  necessary  to  be  learnt  before  one  can  understand  the  efi 
fects  resulting  from  the  earth's  motions  ' 


104  ON  THE  EARTH. 

Stand.  Had  you  been  early  made  acquainted  with  the 
terms  which  ^relate  to  figure  and  motion,  how  much  it 
would  have  facilitated  your  progress  in  natural  philoso- 
phy !  I  have  been  obliged  to  confine  myself  to  the  most 
common  and  familiar  expressions,  in  explaining  the  laws 
of  nature,  though  I  am  convinced  that  appropriate  and 
scientifick  terms  would  have  conveyed  more  precise  and 
accurate  ideas  ;  but  I  was  afraid  of  not  being  understood. 

Eniilij.  You  may  depend  upon  our  learning  the  names 
of  these  lines  thoroughly,  Mrs.  B. ;  but  before  we  com- 
mit them  to  memory,  will  you  have  the  goodness  to  ex- 
plain them  to  us  ? 

Mrs.  B.  Most  willingly.  This  globe,  or  sphere, 
represents  the  earth  ;  the  line  which  passes  through  its 
centre,  and  on  which  it  turns,  is  called  its  axis,  and  the 
two  extremities  of  the  axis  A  and  B,  are  the  poles,  dis- 
tinguished by  the  names  of  the  north  and  south  pole. 
The  circle  C  D,  which  divides  the  globe  into  two  equal 
parts  between  the  poles,  is  called  the  equator,  or  equi- 
noctial line  ;  that  part  of  the  globe  to  the  north  of  the 
equator  is  the  northern  hemisphere  ;  that  part  to  the 
south  of  the  equator,  the  southern  hemisphere.  The 
small  circle  E  F,  which  surrounds  the  north  pole,  is  call- 
ed the  arctick  circle  ;  that  G  H,  which  surrounds  the 
south  pole,  the  antarctick  circle.  There  are  two  inter- 
mediate circles  between  the  polar  circles  and  the  equator  ; 
that  to  the  north,  I  K,  called  the  tropick  of  Cancer  ;  that 
to  the  south,  L  M,  called  the  tropick  of  Capricorn. 
Lastly,  this  circle,  L  K,  which  divides  the  globe  into 
two  equal  parts,  crossing  the  equator  and  extending 
northward  as  far  as  the  tropick  of  Cancer,  and  southward 
as  far  as  the  tropick  of  Capricorn,  is  called  the  ecliptick. 
The  delineation  of  the  ecliptick  on  the  terrestrial  globe  is 
not  without  danger  of  conveying  false  ideas  ;  for  the 
ecliptick  (as  I  have  before  said)  is  an  imaginary  circle  in 
the  heavens  passing  through  the  middle  of  the  zodiack,  and 
situated  in  the  plane  of  the  earth's  orbit. 

403.     Wliat,  in  an  artificial  globe,  represents  the  earth's  axis? 

404.     What  are  the  extremities  of  the  axis  called  ? 405. 

What  is  the  equator  ? 406.     What  line  in  the  figure  represents 

the  equator  ? — What  ones  the  Tropicks  .■' — What  ones  the  Polar 

Circles  ? — What  one  the   Ecliptick  ? 407.     By  what  name  are 

the  two  tropicks  distinrruished   from    each    other  ? 408      By 

what  name  are  the  polar  circles  distinjjuished  from  each  other  ? 
409.     Where  is  the  ecliptick  situated  ? 


ON  THE  EARTH.  105 

Caroline*  I  do  not  understand  the  meaning  of  the 
plane  of  the  earth's  orbit. 

Mrs,  B,  A  plane,  or  plain,  is  an  even  level  surface. 
Let  us  suppose  a  smooth  thin  solid  plane  cutting  the  sun 
through  the  centre,  extending  out  as  far  as  the  fixed 
stars,  and  terminating  in  a  circle  which  passes  through 
the  middle  of  the  zodiack  ;  in  this  plane  the  earth  would 
move  in  its  revolution  round  the  sun  ;  it  is  therefore 
called  the  plane  of  the  earth's  orbit,  and  the  circle  in 
which  this  plane  cuts  the  signs  of  the  zodiack  is  the  eclip- 
tick.  Let  the  fig.  1.  plate  IX.  represent  such  a  plane,  S 
the  sun,  E  the  earth  with  its  orbit,  and  A  B  C  D  the 
ecliptick  passing  through  the  middle  of  the  zodiack. 

Emily.  If  the  ecliptick  relates  only  to  the  heavens, 
why  is  it  described  upon  the  terrestrial  globe  1 

Mrs,  B.  It  is  convenient  for  the  demonstration  of  a 
variety  of  problems  in  the  use  of  the  globes  ;  and  besides, 
the  obliquity  of  this  circle  to  the  equator  is  rendered  more 
conspicuous  by  its  being  described  on  the  same  globe; 
and  the  obliquity  of  the  ecliptick  shows  the  inclination  of 
the  earth's  axis  to  the  plane  of  its  orbit.  But  to  return 
to  figr.  2.  plate  VIII. 

The  spaces  between  the  several  parallel  circles  on  the 
terrestrial  globe  are  called  zones  ;  that  which  is  compre- 
hended between  the  tropicks  is  distinguished  by  the  name 
of  the  torrid  zone ;  the  spaces  which  extend  from  the 
tropicks  to  the  polar  circles,  the  north  and  south  tempe- 
rate zones  ;  and  the  spaces  contained  within  the  polar 
circles,  the  frigid  zones. 

The  several  lines  which,  you  observe,  are  drawn  from 
one  pole  to  the  other,  cutting  the  equator  at  right  angles, 
are  called  meridians.  When  any  one  of  these  meridians 
is  exactly  opposite  the  sun  it  is  mid-day,  or  twelve  o'clock 
in  the  day,  with  all  the  places  situated  on  that  meridian  ; 
and,  with  the  places  situated  on  the  opposite  meridian,  it 
is  consequently  midnight. 

410.  What  is  to  be  understocd  by  the  plane  of  the  earth's  orbit  ? 
41 1.  By  what  figure  is  it  represented  ? 412.  If  the  eclip- 
tick relate  only  to  the  heavens,  why  is  it  described  on  the  ter- 
restrial   globe  ? 413.     What    are    called    the    zones  ^ 414. 

Where  is  the  torrid  zone  ? 415.     Where  arc    the  temperate 

zones  ? 41G.    Where  are  the  frigid  zones  ? 417.     What  are 

the  meridian  lines  ? 418.     When  is  it  twelve  o'clock  at  noon 

to  all  places  under  any  particular  meridian  ? 419.     To  what 

places  will  it,  at  the  same  time,  be  midnight  ? 


106  ON  THE  EARTH. 

Emily,  To  places  situated  equally  distant  from  these 
two  meridians,  it  must  then  be  six  o'clock  ? 

Mrs,  B.  Yes ;  if  they  tire  to  the  east  of  the  sun's 
meridian  it  is  six  o'clock  in  the  afternoon,  because  the 
sun  will  have  previously  passed  over  them  ;  if  to  the 
west,  it  is  six  o'clock  in  the  morning,  and  the  sun  will 
be  proceeding  towards  that  meridian. 

Those  circles  which  divide  the  globe  into  two  equal 
parts,  such  as  the  equator  and  the^ecliptick,  are  called 
greater  circles  ;  to  distinguish  them  from  those  which  di- 
vide it  into  two  unequal  parts,  as  the  tropicks  and  polar 
circles,  wliich  are  called  lesser  circles.  All  circles  are 
divided  into  360  equal  parts,  called  degrees,  and  degrees 
into  60  equal  parts,  called  minutes.  The  diameter  of  a 
circle  is  a  right  line  drawn  across  it,  and  passing  through 
the  centre  ;  for  instance,  the  boundary  of  this  sphere  is 
a  circle,  and  its  axis  the  diameter  of  that  circle  ;  the  di- 
ameter is  equal  to  a  little  less  than  one-third  of  the  cir- 
cuniA:-v<.«oo  Can  you  tell  me  nearly  how  many  decrees 
it  contams  ? 

Caroline,  It  must  be  something  less  than  one-third  of 
360  degrees,  or  nearly  120  degrees. 

Mrs,  B,  Right ;  now  Emily  you  may  tell  me  exactly 
how  many  degrees  are  contained  in  a  meridian  ? 

Emily,  A  meridian,  reaching  from  one  pole  to  the 
other,  is  half  a  circle,  and  must  therefore  contain  180 
degrees. 

Mrs.  B.  Very  well ;  and  what  number  of  degrees  are 
there  from  the  equator  to  the  poles  1 

Caroline,  The  equator  being  equally  distant  from 
either  pole,  that  distance  must  be  half  of  a  meridian,  or 
a  quarter  of  the  circumference  of  a  circle,  and  contain  90 
degrees. 

Mrs,  B,  Besides  the  usual  division  of  circles  into  de- 
grees,   the    ecliptick    is    divided    into    12    equal    parts, 

420.     To  what  places  will  it  be  six  o'clock  in  the  morning,  and 

to  what  ones  six  in  the  evening  ? 421.     What  circles  are  called 

greater  circles  ? 422.     What  ones    are  called  lesser   circles  ^ 

423.     Into  how  many  parts  are  all  circles  divided  .'' 424. 

How  are  degrees  divided  : 425.  What  is  the  diameter  of  a  cir- 
cle ? 426.     How  many  degrees  does  the  diameter  of  a  circle 

contain  ? 427.     How"  many  degrees  are  there    in  a  meridian 

reaching  from  one  pole  to  the  other  .? 428.  How  many  de- 
grees are  there  between  the  equator  and  the  poles  .'—429.  How 
is  the  ecliptick  divided  ? 


ON  THE  EARTH-  107 

called  signs,  which  bear  the  names  of  the  constellations 
throdgh  which  this  circle  passes  in  the  heavens.  The 
degrees  measured  on  the  meridians  from  north  to  south, 
or  south  to  north,  are  called  degrees  of  latitude  ;  those 
measured  from  east  to  west  on  the  equator,  the  ecliptick, 
or  any  of  the  lesser  circles,  are  called  degrees  of  longi- 
tude ;  hence  these  circles  bear  the  name  of  longitudinal 
circles ;  they  are  also  called  parallels  of  latitude. 

Eniihj.  The  degrees  of  longitude  must  then  vary  in 
length  according  to  the  dimensions  of  the  circle  on  which 
they  are  reckoned  ;  those,  for  instance,  at  the  polar  cir- 
cles will  be  considerably  smaller  than  those  at  the  equa- 
tor ? 

3Irs.  B.  Certainly  ;  since  the  degrees  of  circles  of 
different  dimensions  do  not  vary  in  number,  they  must 
necessarily  vary  in  length.  The  degrees  of  latitude,  you 
may  observe,  never  vary  in  length  ;  for  the  meridians  on 
which  they  are  reckoned  are  all  of  the  same  dimensions. 

Emily.     And  of  what  lenojth  is  a  degree  of  latitude  ? 

Mrs,  IB.  Sixty  geographical  miles,  which  is  equal  to 
69^  English  statute  miles. 

Emily,  The  degrees  of  longitude  at  the  equator  must 
then  be  of  the  same  dimensions  ? 

Mrs,  B,  They  would,  were  the  earth  a  perfect  sphere  ; 
but  its  form  is  not  exactly  spherical,  being  somewhat 
protuberant  about  the  equator,  and  flattened  towards  the 
poles.  This  form  is  supposed  to  proceed  from  the  superi- 
our  action  of  the  centrifugal  power  at  the  equator. 

Caroline.  I  thought  I  had  understood  the  centrifugal 
force  perfectly,  but  I  do  not  comprehend  its  effect  in  this 
instance. 

Mrs.  B.  You  know  that  the  revolution  of  the  earth 
on  its  axis  must  give  every  particle  a  tendency  to  fly  off 
from  the  centre,  that  this  tendency  is  stronger  or  weaker 
in  proportion  to  the  velocity  with  which  the  particle 
moves  ;  now  a  particle  situated  near  one  of  the  polar 
circles  makes  one  rotation  in  the  same  space  of  time  as  a 


430.     What  is  latitude  P 431.     What  is  longitude  ? 432. 

Are  the  degrees  of  long-itude  in  different  latitudes   of  the  same 

length  ? -433.     What  is  the  length  of  a  degree  of  latitude  .= 

431.  What  is  the  reason  that  a  degree  of  longitude  on  the  equa- 
tor is  not  the  same  as  a  degree  of  latitude  ? 435.  What  occa- 
sions the  protuberance  of  the  earth  at  the  equator  ^ 


108  ON  THE  EARTH. 

particle  at  the  equator  ;  the  latter,  therefore,  having  a 
much  larger  circle  to  describe,  travels  proportionally 
faster,  consequently  the  centrifugal  force  is  much  stronger 
at  the  equator  than  at  the  polar  circles  :  it  gradually  de- 
creases as  you  leave  the  equator  and  approach  the  poles, 
where,  as  there  is  no  rotatory  motion,  it  entirely  ceases. 
Supposing,  therefore,  the  earth  to  have  been  originally 
in  a  fluid  state,  the  particles  in  the  torrid  zone  would  re- 
cede much  further  from  the  centre  than  those  in  the  frigid 
zones  ;  thus  the  polar  regions  would  become  flattened, 
and  those  about  the  equator  elevated. 

Caroline,  I  did  not  consider  that  the  particles  in  the 
neighbourhood  of  the  equator  move  with  greater  velocity 
than  those  about  the  poles  ;  this  was  the  reason  I  could 
not  understand  you. 

Mrs,  B.  You  must  be  careful  to  remember,  that  those 
parts  of  a  body  which  are  furthest  from  the  centre  of  mo- 
tion must  move  with  the  greatest  velocity  :  the  axis  of 
the  earth  is  the  centre  of  its  diurnal  motion,  and  the  equa- 
torial regions  the  parts  most  distant  from  the  axis. 

Caroline.  My  head  then  moves  faster  than  my  feet ; 
and  upon  the  summit  of  a  mountain  we  are  carried  round 
quicker  than  in  a  valley  1 

Mrs,  B,  Certainly,  your  head  is  more  distant  from 
the  centre  of  motion,  than  your  feet ;  the  mountain-top 
than  the  valley  :  and  the  more  distant  any  part  of  a  body 
is  from  the  centre  of  motion,  the  larger  is  the  circle  it 
will  describe,  and  the  greater  therefore  must  be  its  ve- 
^locity. 

Emily,  I  have  been  reflecting  that  if  the  earth  is  not 
a  perfect  circle — ^- 

Mrs,  B,  A  sphere  you  mean,  my  dear  ;  a  circle  is  a 
round  line,  every  part  of  which  is  equally  distant  from  the 
centre  ;  a  sphere  or  globe  is  a  round  body,  the  surface  of 
which  is  every  where  equally  distant  from  the  centre. 

Emily,  If  then,  the  earth  is  not  a  perfect  sphere, 
but  prominent  at  the  equator,  and  depressed  at  the  poles, 
would  not  a  body  weigh  heavier  at  the  equator  than  at 
the  poles  ?  For  the  earth  being  thicker  at  the  equator, 

436.  In  what  manner  can  you  account  for  this  protuberance 
from  centrifugal  motion  ? 437.  Why  does  the  head  of  a  per- 
son move  faster  than  his  feet  in  the  revolution  of  the  earth  upon 
its  axis  ? 438.     What  is  a  sphere  or  globe  ? 


t)N  THE  EARTH,  109 

the  attraction  of  gravity  perpendicularly  downwards  must 
be  stronger. 

3Irs.  B,  Your  reasoning  has  some  plausibility,  but  I 
am  sorry  to  be  obliged  to  add  that  it  is  quite  erroneous ; 
for  the  nearer  any  part  of  the  surface  of  a  body  is  to  the 
centre  of  attraction,  the  more  strongly  it  is  attracted  ; 
because  the  most  considerable  quantity  of  matter  is  about 
that  centre.  In  regard  to  its  effects,  you  might  consider 
the  power  of  gravity,  as  that  of  a  magnet  placed  at  the 
centre  of  attraction. 

Emily,  But  were  you  to  penetrate  deep  into  the 
earth,  would  gravity  increase  as  you  approached  the 
centre  ? 

Mrs.  B.  Certainly  not ;  I  am  referring  only  to  any 
situation  on  the  surface  of  the  earth.  Were  you  to  pene- 
trate into  the  interiour,  the  attraction  of  the  parts  above 
you  would  counteract  that  of  the  parts  beneath  you,  and 
consequently  diminish  the  power  of  gravity  in  proportion 
as  you  approached  the  centre  ;  and  if  you  reached  that 
point,  being  equally  attracted  by  the  parts  all  around 
you,  gravity  would  cease,  and  you  would  be  without 
weight. 

Emihj.  Bodies  then  should  weigh  less  at  the  equator 
than  at  the  poles,  since  they  arp  more  distant  from  the 
centre  of  gravity  in  the  former-  than  in  the  latter  situation. 

Mrs.  B.  And  this  is  r^^ally  the  case  ;  but  the  diffe- 
rence of  weight  would  ^e  scarcely  sensible,  were  it  not 
augmented  by  another  circumstance. 

Caroline,  An*^  what  is  this  singular  circumstance 
which  seems  io  disturb  the  laws  of  nature  ? 

Mrs.  B.  One  that  you  are  well  acquainted  with,  as 
conducing  more  to  the  preservation  than  the  destruction 
of  order, — the  centrifugal  force.  This  we  have  just  ob- 
served to  be  stronger  at  the  equator  ;  and  as  it  tends  to 
drive  bodies  from  the  centre,  it  is  necessarily  opposed  to, 
and  must  lessen  the  power  of  gravity,  which  attracts 
them  towards  the  centre.     We  accordingly  find  that  bo- 


439.     Will  any  body  weigh  the  same  at  the  equator  as  at  the 

poles  ? 440.     Were  one  to    penetrate  deep   into    the    earth, 

would  the  force  of  gravity  increase  ? 441.     Why  not .'' 442. 

Where  will  bodies  weigh  most,  at  the  equator  or  poles  ? 443. 

What  besides  the  protuberance  at  the  equator  causes  bodies  tQ 
weigh  less  there  than  at  the  poles  ? 
10 


110  ON  THE  EARTH. 

dies  weigh  lightest  at  the  equator,  where  the  centrifugal 
force  is  greatest ;  and  heaviest  at  the  poles,  where  this 
power  is  least.* 

Caroline.  Has  the  experiment  been  made  in  these 
different  situations  ? 

Mrs.  B,  Louis  XIV.,  of  France,  sent  philosophers 
both  to  the  equator  and  to  Lapland  for  this  purpose  ;  the 
severity  of  the  climate,  and  obstruction  of  the  ice,  have 
hitherto  rendered  every  attempt  to  reach  the  pole  abor- 
tive ;  but  the  difference  of  gravity  at  the  equator  and  in 
Lapland  is  very  perceptible. 

Caroline.  Yet  I  do  not  comprehend,  how  the  diffe- 
rence of  weight  could  be  ascertained  ;  for  if  the  body  un- 
der trial  decreased  in  weight,  the  weight  which  w^as  op- 
posed to  it  in  the  opposite  scale  must  have  diminished  in 
the  same  proportion.  For  instance,  if  a  pound  of  sugar 
did  not  weigh  so  heavy  at  the  equator  as  at  the  poles,  the 
leaden  pound  which  served  to  weigh  it,  would  not  be  so 
heavy  either  :  therefore  they  would  still  balance  each 
other,  and  the  different  force  of  gravity  could  not  be  as- 
certained by  this  means. 

Mrs.  B.  Your  observation  is  perfectly  just :  the  diffe- 
rence of  gravity  of  bodies  situated  at  the  poles  and  at 
the  equator  cannot  be  ^»scertained  by  weighing  them ;  a 
pendulum  was  therefore  us^d  for  that  purpose. 

Caroline.  What,  the  pendUum  of  a  clock  ?  how  could 
that  answer  the  purpose  ? 

Mrs.  B.  A  pendulum  consists  <^i  a  line,  or  rod,  to 
one  end  of  which  a  weight  is  attached,  -and  it  is  suspend- 
ed by  the  other  to  a  fixed  point,  about  wl«ch  it  is  made 


*  If  the  diurnal  motion  of  the  earth  round  its  axis  vrere  about 
17  times  faster  than  it  is,  the  centrifugal  force  would,  at  the  equa- 
tor, be  equal  to  the  power  of  gravity,  and  all  bodies  there  would 
entirely  lose  weight.  But  if  the  earth  revolved  still  quicker  than 
this,  they  would  all  fly  off. 


444.  How  much  faster  must  the  earth  move  than  it  now  does  to 
have  the  centrifugal  force  balance  that  of  gravity^  and  thereby 
cav^e  bodies  entirely  to  lose  their  iceight  ? 445.  Has  an  at- 
tempt ever  been  made  to  ascertain  whether  bodies  will  weigh  hea- 
vier at  the  poles  than  at  the  equator  ? 446.     By  whom  was  it 

made  ? 447.     Could  the  experiment  be  made  by  the  common 

scales  ? 448.  Why  not  ? 449.  What  instrument  was  used  in 

the  experiment  ? 450.     How  would  you  describe  a  pendulum  f 


ON  THE  EARTH.  Ill 

to  vibrate.  Without  being  put  in  motion,  a  pendulum, 
like  a  plumb  line,  hangs  perpendicular  to  the  general  sur- 
face of  the  earth,  by  which  it  is  attracted  ;  but,  if  you 
raise  a  pendulum,  gravity  will  bring  it  back  to  its  perpen- 
dicular position.  It  will,  however,  not  remain  stationary 
there,  for  the  velocity  it  has  received  durmg  its  descent 
will  impel  it  onwards,  and  it  will  rise  on  the  opposite  side 
to  an  equal  height ;  from  thence  it  is  brought  back  by 
gravity,  and  again  driven  by  the  impulse  of  its  velocity. 

Caroline,  If  so,  the  motion  of  a  pendulum  would  be 
perpetual,  and  I  thought  you  said  that  there  was  no  per- 
petual motion  on  the  earth. 

Mrs.  B.  The  motion  of  a  pendulum  is  opposed  by  the 
resistance  of  the  air  in  which  it  vibrates,  and  by  the  fric- 
tion of  the  part  by  which  it  is  suspended  ;  were  it  possible 
to  remove  these  obstacles,  the  motion  of  a  pendulum 
would  be  perpetual,  and  its  vibrations  perfectly  regular  ; 
being  of  equal  distances,  and  performed  in  equal  times.* 

Einihj.  That  is  the  natural  result  of  the  uniformity  of 
the  power  which  produces  these  vibrations,  for  the  force 
of  gravity  being  always  the  same,  the  velocity  of  the  pen- 
dulum must  consequently  be  uniform. 

Caroline,  No,  Emily,  you  are  mistaken  ;  the  cause  is 
not  always  uniform,  and  therefore  the  effect  will  not  be  so 
either.  I  have  discovered  it,  Mrs.  B. :  since  the  force  of 
gravity  is  less  at  the  equator  than  at  the  poles,  the  vibra- 
tions of  the  pendulum  will  be  slower  at  the  equator  than 
at  the  poles. 


*  The  vibrations  of  pendulums  are  subject  to  many  irregularities 
for  which  no  effectual  remedy  has  yet  been  devised.  These  are 
owing  partly  to  the  variable  density  and  temperature  of  the  air, 
partly  to  the  ricridily  and  friction  of  the  rod  by  which  they  are  sus- 
pended, and  principally  to  the  dilatation  and  contraction  of  the  ma- 
terials, of  which  ihey  are  formed.  The  metalline  rods  of  pendu- 
lums are  expanded  by  heat,  and  contracted  by  cold  ;  therefore 
clocks  will  go  faster  in  winter,  and  slower  in  summer.  The  com- 
mon remedy  for  this  inconvenience  is  the  raising  or  lowering  the 
bob  of  the  pendulum,  by  means  of  a  screw,  as  occasion  may  re- 


quire 


451 .    What  causes  the  vibrations  of  a  pendulum  ? 452.    Why 

are  not  its  vibrations  perpetual  ? 453.     To  what  is  the  irregu- 

laritij  in  the  vibrations  of  pendulums  oioing  f 454.      Why  will 

clocks  go  faster  in  winter  than  in  summer  ? 455.     Where  do 

pendulums  of  the  same  length  vibrate  fastest  ^ 


118 


ON  THE  EARTH. 


Mrs,  B.  You  are  perfectly  right,  Caroline  ;  it  was 
by  this  means  that  the  difference  of  gravity  was  discover- 
ed, and  the  true  figure  of  the  earth  ascertained. 

Emily,  But  how  do  they  contrive  to  regulate  their 
tirne  in  the  equatorial  and  polar  regions  ?  for,  since  in 
this  part  of  the  earth  the  pendulum  of  a  clock  vibrates 
exactly  once  in  a  second,  if  it  vibrates  faster  at  the  poles 
and  slower  at  the  equator,  the  inhabitants  must  regulate 
their  clocks  in  a  different  manner  from  ours. 

Mrs,  B,  The  only  alteration  required  is  to  lengthen 
the  pendulum  in  one  case,  and  to  shorten  it  in  the  other ; 
for  the  velocity  of  the  vibrations  of  a  pendulum  depends 
on  its  length  ;  and  when  it  is  said,  that  a  pendulum  vi- 
brates quicker  at  the  pole  than  at  the  equator,  it  is  sup- 
posing it  to  be  of  the  same  length.  A  pendulum  which 
vibrates  a  second  in  this  latitude  is  36^  inches  long.  In 
order  to  vibrate  at  the  equator  in  the  same  space  of  time, 
it  must  be  lengthened  by  the  addition  of  a  few  lines  ; 
and  at  the  poles,  it  must  be  proportionally  shortened.* 

I  shall  now,  I  think,  be  able  to  explain  to  you  the  va- 
riation of  the  seasons,  and  the  difference  of  the  length  of 
the  days  and  nights  iri  those  seasons  ;  both  effects  result- 
ing from  the  same  cause. 

'  In  moving  round  the  sun,  the  axis  of  the  earth  is  not 
perpendicular  to  the  plane  of  its  orbit.  Supposing  this 
round  table  to  represent  the  plane  of  the  earth's  orbit,  and 
this  little  globe,  which  has  a  wire  passing  through  it,  re- 
presenting the  axis  and  poles,  we  shall  call  the  earth  ;  in: 
moving  round  the  table,  the  wire  is  not  perpendicular  ta 
it,  but  oblique, 


*  What  is  here  stated  concerning  the  length  of  pendulums  as 
connected  with  the  force  of  gravity  i«  at  complete  variance  with 
fact.  The  force  of  gravitation  is  greater,  it  is  well  known,  at  the 
poles  than  at  the  equator  ;  and  since  the  vibration  of  pendulums 
is  occasioned  by  gravity,  the  lengths  of  pendulums  vibrating  in  the 
same  time  must  evidently  be  proportioned  to  the  gravities  at  th& 
places  where  they  vibrate.  Accordingly,  it  is  found,  by  observa- 
tion, in  order  to  vibrate,  at  the  equator,  in  the  same  space,  the 
pendulum  must  not  be  lengthened,  as  above  stated,  but  shortened  ; 
and  at  the  poles,  it  must  not  be  shortened,  but  proportionally 
lengthened. 

456.  How  do  the  pendulums  used  at  the  equator  and  at  the  polar 
regions  compare  in  length  in  order  to  vibrate  in  the  same  time  f 


ON  THE  EARTH.  113 

Emily.  Yes,  I  understand  the  earth  does  not  go  round 
the  sun  in  an  upright  position,  its  axis  is  slanting  or  ob- 
lique. 

Mrs.  B,  All  the  lines,  which  you  learnt  in  your  last 
lesson,  are  delineated  on  this  little  globe  ;  you  must  con- 
sider the  ecliptick  as  representing  the  plane  of  the  earth  s 
orbit ;  and  the  equator  which  crosses  the  ecliptick  in  two 
places,  shows  the  degree  of  obliquity  of  the  axis  of  the 
earth  in  that  orbit,  which  is  exactly  23^  degrees.  The 
points  in  which  the  ecliptick  intersects  the  equator  are  call- 
ed nodes. 

But  I  believe  I  shall  make  this  clear  to  you  by  revolv- 
ing the  little  globe  round  a  candle,  which  shall  represent 
the  sun.     (Plate  IX.  fig.  2.) 

As  I  now  hold  it,  at  A,  you  see  it  in  the  situation  in 
which  it  is  in  the  midst  of  summer,  or  what  is  called  the 
summer  solstice,  which  is  on  the  21st  of  June. 

Emily,  You  hold  the  wire  awry,  I  suppose,  in  order 
to  show  that  the  axis  of  the  earth  is  not  upright  ? 

Mrs.  B.  Yes  ;  in  summer,  the  north  pole  is  inclined 
towards  the  sun.  In  this  season,  therefore,  the  northern 
hemisphere  enjoys  much  more  of  his  rays  than  the  south- 
ern. The  sun,  you  see,  now  shines  over  the  whole  of  the 
north  frigid  zone,  and  notwithstanding  the  earth's  diur- 
nal revolution,  which  I  imitate  by  twirling  the  ball  on 
the  wire,  it  will  continue  to  shine  upon  it  as  long  as  it 
remains  in  this  situation,  whilst  the  south  frigid  zone  is 
at  the  same  time  completely  in  obscurity. 

Caroline.  That  is  very  strange  :  I  never  before  heard 
that  there  was  constant  day  or  night  in  any  part  of  the 
world  !  How  much  happier  the  inhabitants  of  the  north 
frigid  zone  n^ust  be  than  those  of  the  southern  ;  the  first 
enjoy  uninterrupted  day,  while  the  last  are  involved  in 
perpetual  darkness. 

Mrs.  B.  You  judge  with  too  much  precipitation  ;  ex- 
amine a  little  further,  and  you  will  find,  that  the  two 
frigid  zones  share  an  equal  fate. 

457.  What  causes  the  variation  of  the  seasons  and  the  diffe- 
rence of  the  length  of  the  days  and  nights  ? 458.     How  much  is 

the   axis   of  the  earth    inclined  to   the    plane   of  its  orbit  ? 

459.     What  are  the  points  called  where  the  ecliptick  intersects  the 

equator  ? 460.     When'  does  the  summer  solstice  take  place  ? 

— - — 461.     By  which  figure  is  the  change  of  seasons  illustrated  ? 

462.     When  is  the  north  pole  inclined  towards  the  sun  ? 

463.     What  is  the  situation  of  the  south  pole  when  the  north  pole 
is  inclined  to  the  sun  ? 

10* 


114  ON  THE  EARTH. 

We  shall  now  make  the  earth  set  off  from  its  position 
in  the  summer  solstice,  and  carry  it  round  the  sun ;  ob- 
serve that  the  pole  is  always  inclined  in  the  same  direc- 
tion, and  points  to  the  same  spot  in  the  heavens.  There 
is  a  fixed  star  situated  near  that  spot,  which  is  hence 
called  the  North  Polar  star.  Now  let  us  stop  the  earth 
at  B,  and  examine  it  in  its  present  situation ;  it  has  gone 
through  one  quarter  of  its  orbit,  and  is  arrived  at  that 
point  at  which  the  ecliptick  cuts  or  crosses  the  equator, 
a.nd  which  is  called  the  autumnal  equinox. 

Emily,     That  is  then  one  of  the  nodes. 

The  sun  now  shines  from  one  pole  to  the  other,  just  as 
it  would  constantly  do,  if  the  axis  of  the  earth  were  per- 
pendicular to  its  orbit. 

Mrs,  B,  Because  the  inclination  of  the  axis  is  now 
neither  towards  the  sun  nor  in  the  contrary  direction  ;  at 
this  period  of  the  year,  therefore,  the  days  and  nights  are 
equal  in  every  part  of  the  earth.  But  the  next  step  she 
takes  in  her  orbit,  you  see,  involves  the  north  pole  in  dark- 
ness, whilst  it  illumines  that  of  the  south  ;.  this  change 
was  gradually  preparing  as  I  moved  the  earth  from  sum- 
mer to  autumn ;  the  arctick  circle,  which  was  at  first  en- 
tirely illumined,  began  to  have  short  nights,  which  in- 
creased as  the  earth  approached  the  autumnal  equinox; 
and  the  instant  it  passed  that  point,  the  long  night  of  the 
north  pole  commences,  and  the  south  pole  begins  to  enjoy 
the  light  of  the  sun.  We  shall  now  make  the  earth  pro- 
ceed in  its  orbit,  and  you  may  observe  that  as  it  advances, 
the  days  shorten,  and  the  nights  lengthen,  throughout  the 
northern  hemisphere,  until  it  arrives  at  the  winter  solstice, 
on  the  21st  of  December,  when  the  north  frigid  zone  is 
entirely  in  darkness,  and  the  southern  has  uninterrupted 
day-light. 

Caroline,  Then  after  all,  the  sun,  which  I  thought  so 
partial,  confers  his  favours  equally  on  all. 

Mrs,  B,  You  mistake  :  the  inhabitants  of  the  torrid 
z;one  have  much  more  heat  than  we  have,  as  the  sun's 
rays  fall  perpendicularly  on  them,  while  they  shine  ob- 

464 .     To  what  part  of  the  heavens  does  the  north  pole  always 

point  ? 4G5.     What  part  of  the  figure  represents  the  earth  at 

the  autumnal  equinox  ? 466.     How  does  the  sun  shine  upon 

the  earth  at  this  season  of  the  year  ?         467.     When  is  the  winter 

solstice  ? 468.     Why  is  the  heat  of  the  sun  greater  at  tho 

equator  than  at  a  distance  from  it  .'* 


ON  THE  EARTH.  115 

liquely  on  the  rest  of  the  world,  and  almost  horizontally 
on  the  poles  ;  for  during  their  long  day  of  six  months,  the 
sun  moves  round  their  horizon  without  either  rising  or 
setting ;  the  only  observable  difference  is,  that  it  is  more 
elevated  by  a  few  degrees  at  mid-day,  than  at  mid-night. 

Emily.  To  a  person  placed  in  the  temperate  zone,  in 
the  situation  in  which  we  are  in  England,  the  sun  will 
shine  neither  so  obliquely  as  it  does  on  the  poles,  nor  so 
vertically  as  at  the  equator  ;  but  its  rays  will  fall  upon 
him  more  obliquely  in  autumn  and  winter,  than  in  summer. 

Caroline*  And  therefore,  the  inhabitants  of  the  tem- 
perate zones  will  not  have  merely  one  day  and  one  night 
in  the  year  as  happens  at  the  poles,  nor  will  they  have 
equal  days  and  equal  nights  as  at  the  equator  ;  but  their 
days  and' nights  will  vary  in  length,  at  different  times  of 
the  year,  according  as  their  respective  poles  incline  to- 
wards or  from  the  sun,  and  the  difference  will  be  greater 
in  proportion  to  their  distance  from  the  equator. 

Mrs.  B.  We  shall  now  follow  the  earth  through  the 
other  half  of  her  orbit,  and  you  will  observe,  that  now  ex- 
actly the  same  effect  takes  place  in  the  southern  hemi- 
sphere, as  what  we  have  just  remarked  in  the  northern. 
Day  commences  at  the  south  pole  when  night  sets  in  at 
the  north  pole ;  and  in  every  other  part  of  the  southern 
hemisphere  the  days  are  longer  than  the  nights,  while,  on 
the  contrary,  our  nights  are  longer  than  our  days.  When 
the  earth  arrives  at  the  vernal  equinox,  D,  where  the 
ecliptick  again  cuts  the  equator,  on  the  25th  of  March,  she 
is  situated  with  respect  to  the  sun,  exactly  in  the  same 
position,  as  in  the  autumnal  equinox  ;  and  the  only  diffe- 
rence with  respect  to  the  earth,  is,  that  it  is  now  autumn 
in  the  southern  hemisphere,  whilst  it  is  spring  with  us. 

Caroline.  Then  the  days  and  nights  are  again  every 
where  equal  ? 

Mrs.  B.  Yes,  for  the  half  of  the  globe  which  is  en- 
lightened, extends  exactly  from  one  pole  to  the  other, 
the  day  breaks  to  the  north  pole,  and  the  sun  sets  to  the 
south  pole  ;  but  in  every  other  part  of  the  globe,  the  day 
and  night  is  of  twelve  hours'  length,  hence  the  word  equi- 

469.     In  what  direction  do  the  rays  of  the  sun  fall  upon  the 

polar  regions  of  the  earth  ? 470.     When  does  day  commence 

at  the  south  pole  ? 471,     When  does  the  earth  arrive  at  the 

vernal   equinox  ? 472.     What  part  of  the  figure  exhibits  the 

earth  at  the  vernal  equinox  ? 


116  ON  THE  EARTH. 

nox,  which  is  derived  from  the  Latin,  meaning  equal 
night. 

As  the  earth  proceeds  towards  summer,  the  days  length- 
en in  the  northern  hemisphere,  and  shorten  in  the  south- 
ern, till  the  earth  reaches  the  summer  solstice,  when  the 
north  frigid  zone  is  entirely  illumined,  and  the  southern 
is  in  complete  darkness ;  and  we  have  now  brought  the  earth 
again  to  the  spot  from  whence  we  first  accompanied  her. 

Emily,  This  is,  indeed,  a  most  satisfactory  explana- 
tion of  the  seasons  ;  and  the  more  I  learn,  the  more  I  ad- 
mire the  simplicity  of  means  by  which  such  wonderful 
effects  are  produced. 

Mrs.  B.  I  know  not  which  is  most  worthy  of  our 
admiration,  the  cause  or  the  effect  of  the  earth's  revolu- 
tion round  the  sun.  The  mind  can  find  no  object  of 
contemplation,  more  sublime,  than  the  course  of  this  mag- 
nificent globe,  impelled  by  the  combined  powers  of  pro- 
jection and  attraction  to  roll  in  one  invariable  course 
around  the  source  of  light  and  heat :  and  what  can  be 
more  delightful  than  the  beneficent  effects  of  this  vivify- 
ing power  on  its  attendant  planet !  It  is  at  once  the  grand 
principle  which  animates  and  fecundates  nature. 

Einily,  There  is  one  circumstance  in  which  this  little 
ivory  globe  appears  to  me  to  differ  from  the  earth  ;  it  is 
not  quite  dark  on  that  side  of  it,  which  is  turned  from  the 
candle,  as  is  the  case  with  the  earth  when  neither  moon 
nor  stars  are  visible. 

Mrs,  B,  This  is  owing  to  the  light  of  the  candle 
being  reflected  by  the  walls  of  the  room  on  every  part  of  the 
globe,  consequently  that  side  of  the  globe  on  which  the 
candle  does  not  directly  shine,  is  not  in  total  darkness. 
Now  the  skies  have  no  walls  to  reflect  the  sun's  light  on 
that  side  of  our  earth  which  is  in  darkness. 

Caroline,  I  beg  your  pardon,  Mrs.  B.  I  think  that 
the  moon  and  stars  answer  the  purpose  of  walls  in  reflect- 
ing the  sun's  light  to  us  in  the  night. 

Mrs,  B,  Very  well,  Caroline;  that  is  to  say,  the 
moon  and  planets  ;  for  the  fixed  stars,  you  know,  shine 
by  their  own  light. 

Emily,  You  say  that  the  superiour  heat  of  the  equa- 
torial parts  of  the  earth  arises  from  the  rays  falling  perpen- 
dicularly on  those  regions,  whilst  they  fall  obliquely  on 
these  more  northern  regions  ;  now  I  do  not  understand 

472.  Why  are  the  points  where  the  ecliptick  cut3j6r  crosses  the 
equator  called  equinoxes  ? 


ON  THE  EARTH.  117 

why  perpendicular  rays  should  afford  more  heat  than  ob- 
lique rays. 

Caroline*  You  need  only  hold  your  hand  perpendicu- 
larly over  the  candle,  and  then  hold  it  sideways  obliquely, 
to  be  sensible  of  the  difference. 

Emily,  I  do  not  doubt  the  fact,  but  I  wish  to  have  it 
explained. 

Mrs,  B.  You  are  quite  right  ;  if  Caroline  had  not 
been  satisfied  with  ascertaining  the  fact,  without  under- 
standing it,  she  would  not  have  brought  forward  the  can- 
dle as  an  illustration  ;  the  reason  why  you  feel  so  much 
more  heat  if  you  hold  your  hand  perpendicularly  over  the 
candle,  than  if  you  hold  it  sideways,  is  because  a  steam 
of  heated  vapour  constantly  ascends  from  the  candle  or 
any  other  burning  body,  which  being  lighter  than  the  air 
of  the  room,  does  not  spread  laterally  but  rises  perpendi- 
cularly, and  this  led  you  to  suppose  that  the  rays  were  hot- 
ter in  the  latter  direction.  Had  you  reflected,  you  would 
have  discovered  that  rays  issuing  from  the  candle  side- 
ways, are  no  less  perpendicular  to  your  hand  when  held 
opposite  to  them,  than  the  rays  which  ascend  when  your 
hand  is  held  over  them. 

The  reason  why  the  sun*s  rays  afford  less  heat  when 
m  an  oblique  direction  than  when  perpendicular,  is  be- 
cause fewer  of  them  fall  upon  an  equal  portion  of  the 
earth  ;  this  will  be  understood  better  by  referring  to  plate 
X.  fig.  1,  which  represents  two  equal  portions  of  the  sun's 
rays,  shining  upon  different  parts  of  the  earth.  Here  it  is 
evident  that  the  same  quantity  of  rays  fall  on  the  space  A 
B  as  fall  on  the  space  B  C ;  and  as  A  B  is  less  than  B  C, 
the  heat  and  light  will  be  much  stronger  in  the  former  than 
in  the  latter  ;  A  B,  you  see,  represents  the  equatorial  re- 
gions, where  the  sun  shines  perpendicularly  ;  and  B  C,  the 
temperate  and  frozen  climates,  where  his  rays  fall  more 
obliquely.* 

Emily,  This  accounts  not  only  for  the  greater  heat  of 
the  equatorial  regions,  but  for  the  greater  heat  of  summer ; 

*  It  is  well  known,  that  the  south  side  of  a  hill,  in  our  hemisphere, 
is  peculiarly  warm  ;  and  the  north  side,  peculiarly  cold.  This 
is  owing  to  the  different  degrees  of  obliquity,  with  which  the  rays 

473.     Why  is  the  heat  of  perpendicular  rays  more  intense  than 

that  of  oblique  ones  ? 474.     By  which  figure  is  this  illustrated  ? 

475.     How  will  you  explain  Fig.  1,  plate  X.  as  illustrating 

this  subject  ? 476.     Why  is  the  south  side  of  a  hill  icarmer  than 

the  north  side  of  it  ? 


J  18  ON  THE  EARTH. 

as   the   sun   shines   less   obliquely  in   summer    than  in 
winter. 

Mr:i.  jB.  This  you  will  see  exemplified  in  fig.  2,  in 
which  the  earth  is  represented,  as  it  is  situated  on  the  21st 
June,  and  England  receives  less  oblique,  and  consequently 
a  greater  number  of  rays,  than  at  any  other  season  ; 
and  figure  3  shows  the  situation  of  England  on  the  21st 
December,  when  the  rays  of  the  sun  fall  most  obliquely 
upon  her.  But  there  is  also  another  reason  why  oblique 
rays  give  less  heat,  than  perpendicular  rays ;  which  is, 
that  they  have  a  greater  portion  of  the  atmosphere  to  tra- 
verse ;  and  though  it  is  true  that  the  atmosphere  is  itself 
a  transparent  body,  freely  admitting  the  passage  of  the 
sun's  rays,  yet  it  is  always  loaded  more  or  less  with  dense 
and  foggy  vapour,  which  the  rays  of  the  sun  cannot  easily 
penetrate ;  therefore  the  greater  the  quantity  of  atmo- 
sphere the  sun's  rays  have  to  pass  through  in  their  way 
to  the  earth,  the  less  heat  they  will  retain  when  they 
reach  it.  This  will  be  better  understood  by  referring  to 
figure  4.  The  dotted  line  round  the  earth,  describes  the 
extent  of  the  atmosphere,  and  the  lines  which  proceed 
from  the  sun  to  the  earth,  the  passage  of  two  equal  por- 
tions of  the  sun's  rays  to  the  equatorial  and  polar  regions ; 
the  latter,  you  see,  from  its  greater  obliquity  passes  through 
a  greater  extent  of  atmosphere. 

Caroline,  And  this,  no  doubt,  is  the  reason  why  the 
sun  in  the  morning  and  the  evening  gives  so  much  less 
heat,  than  at  mid-day. 

Mrs.  B,  The  diminution  of  heat,  morning  and  even- 
ing, is  certainly  owing  to  the  greater  obliquity  of  the 
sun's  rays ;  and  as  such  they  are  affected  by  both  the 
causes,  which  I  have  just  explained  to  you ;  the  difficul- 
ty of  passing  through  a  foggy  atmosphere  is  perhaps  more 
particularly  applicable  to  them,  as  mists  and  vapours  are 
very   prevalent  about   the  time   of  sunrise  and  sunset. 

of  the  sun  strike  the  different  sides  of  a  hill.  And  a  south-western 
is  warmer  than  a  south  exposure,  because  it  receives  the  sun's  rays 
in  the  warmest  part  of  the  day. 

477.  fVhy  is  a  south-western  exposure  to  the  sun  warmer  than 
a  soiith  exposure  9     478.     What  is  to  be  illustrated  by  Figures 

2  &.  3  of  plate  X. .' 479.     What  is  another  reason  why  obhque 

rays  give  less  heat  than  perpendicular  ones  P 480.     By  which 

figure  is  the  effect  that  the  atmosphere  has  on  the  heat  of  the  sun's 

rays  illustrated  ? 481.     Why  does  the  sun  give  more  heat  at 

mid-dav,  than  in  the  morninir  and  evenini?  - 


ON  THE  EARTH.  119 

But  the  diminished  obliquity  of  the  sun's  rays  is  not  the 
sole  cause  of  the  heat  of  summer  ;  the  length  of  the  days 
greaily  conduces  to  it ;  for  the  longer  the  sun  is  above  the 
horizon,  the  more  heat  he  will  communicate  to  the  earth. 

Caroline,  Both  the  longest  days,  and  the  most  perpen- 
dicular lays,  are  on  the  21st  of  June  ;  and  yet  the  great- 
est heat  prevails  in  July  and  August. 

Mrs.  B.  Those  parts  of  the  earth  which  are  once  heat- 
ed, retain  the  heat  for  some  length  of  time,  and  the  addi- 
tional heat  they  receive,  occasions  an  elevation  of  tem- 
perature, although  the  days  begin  to  shorten,  and  the 
sun's  rays  fall  more  obliquely.  For  the  same  reason,  we 
have  generally  more  heat  at  three  o'clock  in  the  afternoon, 
than  at  twelve  when  the  sun  is  on  the  meridian.* 


*  There  are  also  other  causes  which  have  an  effect  on  tempera- 
ture. When  the  sun's  rays  strike  upon  the  land,  they  are  stop- 
ped and  accumulated  at  the  surface.  They  are  then  reflected 
into  the  air  and  to  surrounding  objects ;  so  that  the  reflected 
heat  is  often  greater  than  the  direct  heat  of  the  sun.  Hence,  the 
heat  in  valleys  vv'here  the  rays  are  reflected  by  the  hills  and  raoun- 
tainSj  is  sometimes  very  great.  In  an  elevated  valley  in  Switzer- 
land, the  heat  is  so  much  increased  by  reflection,  that  in  the  cen- 
tre there  is  a  spot  of  perpetual  verdure,  in  the  midst  of  perpetual 
snows  and  glaciers;  and  there  are  plains  on  the  Himmaleh  moun- 
tains 15,000  feet  above  the  level  of  the  sea,  which  produce  fine 
pasturage  ;  and,  at  the  height  of  11,000  feet,  which  is  above  the 
region  of  perpetual  snows  on  the  Andes,  in  the  same  latitude,  bar- 
ley and  buckv/heat  flourish.  But,  unless  heat  is  thus  increased,  it 
is  reckoned  as  continually  diminishing  as  we  ascend  above  the 
level  of  the  sea,  especially  on  lofiy  mountains,  where  it  is  reflected 
into  the  dry,  clear  air  around  them,  and  is  carried  off  by  the  winds 
which  sweep  over  them,  without  any  opportunity  for  accumula- 
tion. Thus  an  elevation  of  500  yards  produces  the  same  effect  as 
a  distance  of  5,000  miles  from  the  equator.  At  the  height  of 
(3,000  or  8,000  feet  under  thetropicks,  we  find  the  same  climate  as 
in  latitude  49^  in  France.  At  13,000  feet  we  find  the  frosts  of 
the  frigid  zone  ;  and  at  15,730  feet,  the  mountains,  based  upon 
the    most   scorching    plains,   are  capped    with   perpetual   snow. 

482-      What,  besides    the  direction  of  the   sun's  rays,  effects 

the  temperature  of  the  places  where  they  fall  ? 483*     Why  is 

it  warmer  in  July  and  August  than  in  June,  when  the  days  are 

longest ;  and  at  2  and  3P.M.  than  at  noon  ? 484.     Why  is  the 

dcfrrce  of  heat  increased  in  valleijs  ? 485.      What  fact  is  stated 

relating  to  this  suhject  concerning  a  valley  in  Sicitzcrland  ? 

480,      What  facts  are  stated  concerning  the  plains  of  H-mmalehf 

487.     How  is  temperature  effected  in  ascending  above  the 

level    of    the  sea  ? 488.     In  what  ratio,  compared  v/uh  the 

degrees  of  latitude^  does  heat  diminish  in  rising  above  the  level 
of  the  sea  ? 


120  ON  THE  EARTH. 

Emily »  And  pray,  have  the  other  planets  the  same  vi- 
cissitudes of  seasons  as  the  earth  ? 

Mrs,  B,  Some  of  ahem  more,  some  less,  according 
as  their  axes  deviate  more  or  less  from  the  perpendicular 
to  the  plane  of  their  orbits.  The  axis  of  Jupiter  is  near- 
ly perpendicular  to  the  plane  of  his  orbit ;  the  axis  of 
Mars  and  of  Saturn  are  each  inclined  at  angles  of  about 
sixty  degrees  ;  whilst  the  axis  of  Venus  is  believed  to  be 
elevated  only  fifteen  or  twenty  degrees  above  her  orbit ; 
the  vicissitudes  of  her  seasons  must  therefore  be  conside- 
rably greater  than  ours.  For  further  particulars  respect- 
ing the  planets,  I  shall  refer  you  to  Bonnycastle's  Intro- 
duction to  Astronomy. 


When  the  rays  of  the  sun  strike  upon  the  water,  they  will  pene- 
trate GOO  or  700  feet,  if  there  is  that  depth  ;  and  the  heat  will  be 
diffused  through  the  mass,  remainin<r.  till  carried  off  by  evapo- 
ration. Consequently,  in  hot  climates,  the  body  of  the  ocean  is 
much  cooler  than  the  land;  and  in  cold  ones,  it  is  warmer.  Thus 
two  countries  which  abound  with  rivers,  lakes,  and  marshes,  are 
also  less  subject  to  the  extremes  of  heat  and  cold,  than  those  which 
are  dry. 

In  addition  to  the  direct  effects  of  the  sun,  the  different  parts  of 
the  earth  exert  a  continual  influence  on  each  other.  The  deserts 
of  Arabia  and  Africa  are  like  immense  furnaces,  in  increasing  the 
heat  of  all  the  regions  on  the  Mediterranean  sea,  in  the  south  of 
Europe  and  west  of  Asia.  On  the  other  hand,  Siberia  and  the 
northern  portions  of  North  America  have  their  cold  increased  by  po- 
lar winds,  which  are  not  interrupted  by  mountains,  while  Europe 
is  much  protected  from  them  by  its  mountains. 

The  following  may  be  considered  a  rule  for  determining  the 
effect  produced  on  temperature  by  winds.  When  the  prevailing 
winds  to  which  a  country  is  exposed,  come  from  polar  or  elevated 
regions,  the  cold  is  greater  than  the  latitude  would  make  it ;  when 
they  come  from  warmer  regions,  and  especially  from  deserts,  they 
increase  the  heat ;  and  when  they  come  from  the  ocean,  or  large 
bodies  of  water,  they  diminish  both  heat  and  cold,  according  to 
the  climate,  rendering  the  temperature  more  uniform  through  the 
year. 

489.     What  facts  are  mentioned  relating  to  the  rays  of  the  sun 

falling  upon  water,  as  effecting  temperature  ? 400.    What  ones 

are  mentioned  relating  to  the  influence  which  different  portions  of 
the  earth  exercise  npon  each  other,  as  effecting  temperature  9 
491.     What  is  the  rule  named  for  determining  the  effects  -produced 

hy  the  winds  on  temperature  ? 49:2.     Have  the  other   planets 

the   same    vicissitudes   of  seasons,  that   the   earth  has  ? 493. 

Which  planet  has  its  axis  nearly  perpendicular  to  the  plane  of  its 

orbit  ? 494.     How  are  the  axes  of  Mars  and  Saturn  ^ 495. 

How  is  the  axis  of  Venus  ? 


ON  THE  EARTH.  121 

I  have  but  one  more  observation  to  make  to  you  rela- 
tive to  the  earth's  motion,  which  is,  that  although  vi^e 
have  but  305  days  and  nights  in  the  year,  she  performs 
*3Q6  complete  revolutions  on  her  axis  during  that  time. 

Caroline.  How  is  that  possible  ?  for  every  complete 
revolution  must  bring  the  same  place  back  to  the  sun. 
It  is  now  just  twelve  o'clock,  the  sun  is,  therefore,  on 
our  meridian  ;  in  twenty-four  hours  will  it  not  be  return- 
ed to  our  meridian  again  1  and  will  not  the  earth  have 
made  a  complete  rotation  on  its  axis  ? 

Mrs.  B.  If  the  earth  had  no  progressive  motion  in  its 
orbit  whilst  it  revolves  on  its  axis,  this  would  be  the 
case  ;  but  as  it  advances  almost  a  degree  westward  in 
its  orbit,  in  the  same  time  that  it  completes  a  revolution 
eastward  on  its  axis,  it  must  revolve  nearly  one  degree 
more  in  order  to  bring  the  same  meridian  back  to  the  sun. 

Caroline.  Oh,  yes  !  it  will  require  as  much  more  of  a 
second  revolution  to  bring  the  same  meridian  back  to  the 
sun,  as  is  equal  to  the  space  the  earth  has  advanced  in 
her  orbit,  that  is,  nearly  a  degree  ;  this  difference  is, 
jiowever,  very  little. 

Mrs.  B.  These  small  daily  portions  of  rotation  are 
each  equal  to  the  three  hundred  and  sixty-fifth  part  of  a 
circle,  which  at  the  end  of  the  year  amounts  to  one  com- 
plete rotation. 

Emily.  That  is  extremely  curious.  If  the  earth, 
then,  had  no  other  than  its  diurnal  motion,  we  should 
have  366  days  in  the  year. 

Mrs.  B.  We  should  have  366  days  in  the  same  period 
of  time  that  we  now  have  365  ;  but  if  we  did  not  revolve 
round  the  sun,  we  should  have  no  natural  means  of  com- 
puting years. 

You  will  be  surprised  to  hear,  that  if  time  is  calculated 
by  the  stars  instead  of  the  sun,  the  irregularity  which  we 
have  just  noticed  does  not  occur,  and  that  one  complete 
rotation  of  the  earth  on  its  axis,  brings  the  same  meridian 
back  to  any  fixed  star. 

Emily.     That   seems  quite    unaccountable  ;  for    the 


496.     To  what  is  it  owing  that  the  earth  performs  366  revolu- 
tions in  a  year  that  has  but  365  days  and  nights  ? 497.     Under 

what  circumstances  should  we  have  366  days  in  the  same  period 

of  time  that  we  now  have  365  ? 498.    In  what  way  might  time 

be  calculated  so  as  to  avoid  this  irregularity  ? 


122  ON  THE  EARTH. 

earth  advances  in  her  orbit  with  regard  to  the  fixed  stars 
the  same  as  with  regard  to  the  sun. 

3Irs.  B.  True,  but  then  the  distance  of  the  fixed  stars 
is  so  immense,  that  our  solar  system  is  in  comparison  to 
it  but  a  spot,  and  the  whole  extent  of  the  earth's  orbit  but 
a  point ;  therefore,  whether  the  earth  remained  stationary, 
or  whether  it  revolved  in  its  orbit  during  its  rotation  on 
its  axis,  no  sensible  difference  would  be  produced  with 
regard  to  the  fixed  stars.  One  complete  revolution  brings 
the  same  meridian  back  to  the  same  fixed  star  ;  hence  the 
fixed  stars  appear  to  go  round  the  earth  in  a  shorter  time 
than  the  sun  by  three  minutes,  fifty-six  seconds  of  time. 

Caroline.  These  three  minutes,  fifty-six  seconds  is  the 
time  which  the  earth  takes  to  perform  the  additional  three 
hundred  and  sixty-fifth  part  of  the  circle,  in  order  to  bring 
the  same  meridian  back  to  the  sun. 

Mrs.  B,  Precisely.  Hence  the  stars  gain  every  day 
three  minutes,  fifty-six  seconds  on  the  sun,  which  makes 
them  rise  that  portion  of  time  earlier  every  day. 

When  time  is  calculated  by  the  stars  it  is  called  sidereal 
time,  when  by  the  sun,  solar  or  apparent  time.* 

Caroline.  Then  a  sidereal  day  is  three  minutes,  fifty- 
six  seconds  shorter  than  a  solar  day  of  twenty-four  hours. 

Mrs.  B.  I  must  also  explain  to  you  what  is  meant  by  a 
sidereal  year. 

The  common  year,  called  the  solar  or  tropical  year, 
containing  365  days,  five  hours,  forty  eight  minutes,  and 
fifty-two  seconds,  is  measured  from  the  time  the  sun  sets 
out  from  one  of  the  equinoxes,  or  solstices,  till  it  returns 
to  the  same  again ;  but  this  year  is  completed  before  the 
earth  has  finished  one  entire  revolution  in  its  orbit. 


^  If  one  clock  should  be  so  well  regulated  as  to  show  th§  time 
to  be  XII  at  noon  this  day,  and  on  the  365th  day  afterwarcf ;  and 
another  clock  should  be  so  well  regulated  as  to  show  the  time  to 
be  XII  every  day  or  night  when  any  given  star  is  on  the  meridian, 
the  latter  clock  would  gain  three  minutes,  fifty-five  seconds,  and 
fifty-four  sixtieth  parts  of  a  second  upon  the  former  in  each  revolu- 
tion of  the  same  star  to  the  meridian. 

499.     Why  do  the  fixed  stars  appear  to  revolve  round  the  earth 

quicker  than  the  sun  ? 500.     How  much  quicker  than  the  sun 

do  the  fixed  stars  appear  to  go  round  the  earth  ? 501.     WTien 

is  time  called  sidereal,  and  when  solar  or  apparent  time  .'* 502. 

What  illustration  is  given  of  this  in  the  note  ? 503.     What  is 

the  common  or  solar  vear  ' 


ON  THE  EARTH.  123 

Emily.  I  thought  that  the  earth  performed  one  com- 
plete revolution  in  its  orbit  every  year  ;  what  is  the  rea- 
son of  this  variation  ? 

Mrs.  B.  It  is  owing  to  tlie  spheroidal  figure  of  the 
earth.  The  elevation  about  the  equator  produces  much 
the  same  effect  as  if  a  similar  mass  of  matter,  collected  in 
the  form  of  a  moon,  revolved  round  the  equator.  When 
this  moon  acted  on  the  earth  in  conjunction  with  or  in  op- 
position to  the  sun,  variations  in  the  earth's  motion  would 
be  occasioned,  and  these  variations  produce  what  is  called 
the  precession  of  the  equinoxes. 

Emily.  What  does  that  mean  ?  I  thought  the  equi- 
noctial points,  or  nodes,  were  fixed  points  in  the  heavens, 
in  which  the  equator  cuts  the  ecliptick. 

Mrs.  B.  These  points  are  not  quite  fixed,  but  have  an 
apparently  retrograde  motion,  that  is  to  say,  instead  of 
being  every  revolution  in  the  same  place,  they  move  back- 
wards. Thus  if  the  vernal  equinox  is  at  A,  {^g.  I,  plate 
XI.)  the  autumnal  one  v*^ill  be  at  B  instead  of  at  C,  and 
the  following  vernal  equinox  at  D  instead  of  at  A,  as 
would  be  the  case  if  the  equinoxes  were  stationary  at  op- 
posite points  of  the  earth's  orbit. 

Caroline.  So  that  when  the  earth  moves  from  one  equi- 
nox to  the  other,  though  it  takes  half  a  year  to  perform 
the  journey,  it  has  not  travelled  through  half  its  orbit. 

Mrs.  B.  And,  consequently,  when  it  returns  again  to 
the  first  equinox,  it  has  not  completed  the  whole  of  its 
orbit.  In  order  to  ascertain  when  the  earth  has  perform- 
ed an  entire  revolution  in  its  orbit,  we  must  observe  when 
the  sun  returns  in  conjuncdon  with  any  fixed  star  ;  and 
this  is  called  a  sidereal  year.  Supposing  a  fixed  star  si- 
tuated at  E,  {^g,  1,  plate  XI.)  the  sun  would  not  appear  in 
conjunction  with  it  till  the  earth  had  returned  to  A,  when 
it  would  have  completed  its  orbit. 

Emily.  And  how  much  longer  is  the  sidereal  than  the 
solar  year  ? 


504.     What  is  the   reason  that  the  solar  year  is  completed  be- 
fore the  earth  has  made  one  entire  revolution  in  its  orbit  ? 

505.     What  is  called  the  precession  of  the  equinoxes,  and  how  is 

it  produced  ? 506.     How  would  you  explain  the  precession  of 

the  equinoxes  by  the  figure  ? 507.     How  can  it  be  ascertained 

when  the  earth  has  performed  one  entire  revolution  in  its  orbit .' 

; 508.     What  is  a  sidereal  year  ? 509.     How  much  longer 

is  the  sidereal  than  the  solar  year  ? 


1$J4  ON  THE  MOON. 

Mrs,  B.  Only  twenty  minutes ;  so  that  the  variation 
of  the  equinoctial  points  is  very  inconsiderable.  I  have 
given  them  a  greater  extei;it  in  the  figure  in  order  to  ren- 
der them  sensible. 

In  regard  to  time,  I  must  further  add,  that  the  earth's 
diurnal  motion  on  an  inclined  axis,  together  with  its  an- 
nual revolution  in  an  elliptick  orbit,  occasions  so  much 
complication  in  its  motion,  as  to  produce  many  irregula- 
rities ;  therefore,  true  equal  time  cannot  be  measured  by 
the  sun.  A  clock,  which  was  always  perfectly  correct, 
would  in  some  parts  of  the  year  be  before  the  sun,  and  in 
other  parts  after  it.  There  are  but  four  periods  in  which 
the  sun  and  a  perfect  clock  would  agree,  which  is  the 
I5th  of  April,  the  16th  of  June,  the  23d  of  August,  and 
the  24th  of  December. 

Emily,  And  is  there  any  considerable  difference  be- 
tween solar  time  and  true  time  1 

Mrs,  B,  The  greatest  difference  amounts  to  between 
fifteen  and  sixteen  minutes.  Tables  of  equation  are  con- 
structed for  the  purpose  of  pointing  out  and  correcting 
these  differences  between  solar  time  and  equal  or  mean 
time,  which  is  the  denomination  given  by  astronomers  to 
true  time. 

510.     What  are  the  periods,  when  the  sun  and  a  perfect  clock 

agree  ? 511      What  is  the  greatest  difference  between  solar 

time  and  true  time  ?  ^ 


CONVERSATION  IX. 

ON  THE  MOON. 


Of  the  Moon's  Motion ;  Phases  of  the  Moon  ;  Eclipses  of 
the  Moon  ;  Eclipses  of  Jupiter's  Moons  ;  Of  the  Lati- 
tude  and  Longitude  ;  Of  the  Transits  of  the  Inferiour 
Planets;   Of  the  Tides. 


MRS.  B. 


We  shall  to-day  confine  our  attention  to  the  moony 
which  offers  many  interesting  phenomena. 

The  moon  revolves  round  the  earth  in  the  space  of 
about  twenty -nine  days  and  a  half,  in  an  orbit  nearly  pa- 

512.     In  what  time  docs  the  moon  revolve  about  the  earth  ? 


ON  THE  MOON.  125 

rallel  to  that  of  the  earth,  and  accompanies  us  in  our  revo- 
lution round  the  sun. 

Emily,  Her  motion  then  must  be  rather  of  a  compli- 
cated nature ;  for  as  the  earth  is  not  stationary,  but  ad- 
vances in  her  orbit  whilst  the  moon  goes  round  her,  the 
moon  must  proceed  in  a  sort  of  progressive  circle. 

Mr'i.  B.  That  is  true  ;  and  there  are  also  other  cir- 
cumstances which  interfere  with  the  simplicity  and  regu- 
larity of  the  moon's  motion,  but  which  are  too  intricate 
for  you  to  understand  at  present. 

The  moon  always  presents  the  same  face  to  us,  by 
which  it  is  evident  that  she  turns  but  once  upon  her  axis, 
whilst  she  performs  a  revolution  round  the  earth  ;  so  that 
the  inhabitants  of  the  moon  have  but  one  day  and  one 
night  in  the  course  of  a  lunar  month. 

Cwoline,  We  afford  them  however  the  advantage  ofa 
magnificent  pr^on  to  enlighten  their  long  nights. 

Mrs.  B.  That  advantage  is  but  partial ;  for  since  we 
always  see  the  same  hemisphere  of  the  moon,  the  inhabi- 
tants of  that  hemisphere  alone  can  perceive  us. 

Caroline.  One  half  of  the  moon  then  enjoys  our  light 
every  night,  while  the  other  half  has  constantly  nights  of 
darkness.  If  there  are  any  astronomers  in  those  regions, 
they  would  doubtless  be  tempted  to  visit  the  other  hemi- 
sphere, in  order  to  behold  so  grand  a  luminary  as  we 
must  appear  to  them.  But,  pray,  do  they  see  the  earth 
under  all  the  changes  which  the  moon  exhibits  to  us  1 

Mrs,  B,  Exactly  so.  These  changes  are  called  the 
phases  of  the  moon,  and  require  some  explanation.  In 
figure  2,  plate  XI.  let  us  say  that  S  represents  the  sun, 
E  the  earth,  and  A  B  C  D  the  moon  in  different  parts  of 
her  orbit.  When  the  moon  is  at  A,  her  dark  side  being 
turned  towards  the  earth,  we  shall  not  see  her  as  at  a;  but 
her  disappearance  is  of  very  short  duration,  and  as  she  ad- 
vances in  her  orbit  we  perceive  her  under  the  form  of  a 
new  moon  ;  when  she  has  gone  through  one-eighth  of  her 
orbit  at  B,  one  quarter  of  her  eaiirfitened  hemisphere 

513.     In  what  time  does  the  moon  tv.m  on  its  o.xis  ? 514. 

How  is  it  known  how  lon^  it  takes  the  moon  to  revolve  on  its  axis  ? 

515.     What  is  the  length  of  the  days  and  nights  at  the  moon  ? 

516.     Does  the  earth  exhibit  the  same  chjjnges  to  the  moon, 

that  the  moon  exhibits   to  the  earth  ? 517.     What    are   the 

changes  of  the  moon  called .' 18.     How  would  you  explain 

these  changes  by  the  figure  ? 

n* 


126  ON  THE  MOON. 

will  be  turned  towards  the  earth,  and  she  will  then  appear 
horned  as  at  h ;  when  she  has  performed  one  quarter  of 
her  orbit,  she  shows  us  one  half  of  her  enlightened  side  as 
at  f ;  at  d  she  is  said  to  be  gibbous,  and  at  e  the  whole 
of  the  enlightened  side  appears  to  us,  and  the  moon  is  at 
full.  As  she  proceeds  in  her  orbit  she  becomes  again 
gibbous,  and  her  enlightened  hemisphere  turns  gradually 
away  from  us  until  she  completes  her  orbit  and  disap- 
pears, and  then  again  resumes  her  form  of  a  new  moon. 

When  the  moon  is  at  full,  or  a  new  moon,  she  is  said 
to  be  in  conjunction  with  the  sun,  as  they  are  then  both 
in  the  same  direction  with  regard  to  the  earth  ;  when  at 
her  quarters  she  is  said  to  be  in  opposition  to  the  sun. 

Umily,  Are  not  the  eclipses  produced  by  the  moon 
passing  between  the  sun  and  the  earth  ? 

Mrs,  B.  Yes  ;  when  the  moon  passes  between  the 
3un  and  the  earth,  she  intercepts  his  rays,  or  in  other 
words,  casts  a  shadow  on  the  earth,  then  the  sun  is  eclip- 
sed, and  the  day-light  gives  place  to  darkness,  while  the 
moon's  shadow  is  passing  over  us. 

When,  on  the  contrary,  the  earth  is  between  the  sun 
and  the  moon,  it  is  we  who  intercept  the  sun's  rays,  and 
cast  a  shadow  on  the  moon  ;  the  moon  is  then  darkened^ 
she  disappears  from  our  view,  and  is  eclipsed. 

Emily.  But  as  the  moon  goes  round  the  earth  every 
month  she  must  be  once  during  that  time  between  the 
earth  and  the  sun,  and  the  earth  must  likewise  be  once 
between  the  sun  and  the  moon,  and  yet  we  have  not  a 
solar  and  a  lunar  eclipse  every  month. 

Mrs»  B,  The  orbits  of  the  earth  and  moon  are  not 
exactly  parallel,  but  cross  or  intersect  each  other  ;  and 
the  moon  generally  passes  either  above  or  below  the 
earth  w^hen  she  is  in  conjunction  with  the  sun,  and  does 
not  therefore  intercept  the  sun's  rays,  and  produce  an 
eclipse ;  for  this  can  take  place  only  when  the  earth  and 
moon  are  in  conjunction  in  that  part  of  their  orbits  which 


519.     When  is  the  moon  said  to  be  gibbous,  and  when  homed  ? 

520.     When  is  the  moon  said  to  be  in  conjunction  with  the 

sun  ? ^521.     When  is  the  moon  said  to  be  in  opposition  to  the 

sun  ? 522.     What  causes  an  ecUpse  of  the  sun  .'' 523.     How 

is  an  ecTip?e  of  the  moon  caused  ? 521.     As  the  moon  passes 

between  the  sun  and  the  earth,  and  as  the  earth  passes  between  the 
sun  and  the  moon,  once  every  month,  why  do  we  not  have  a  lu- 
nar and  solar  ecUpse  every  month  ? 


ON  THE  MOON.  127 

cross  each  other,  (called  the  nodes  of  their  orbits,)  because 
it  is  then  only,  that  they  are  both  in  a  right  line  with  the 
sun. 

Emily,  And  a  partial  eclipse  takes  place,  I  suppose, 
when  the  moon,  in  passing  by  the  earth,  is  not  sufficiently 
above  or  below  the  earth's  shadow  entirely  to  escape  it  ? 

Mrs,  B,  Yes,  one  edge  of  her  disk  then  dips  into  the 
shadow,  and  is  eclipsed  ;  but  as  the  eartli  is  larger  than 
the  moon,  when  the  eclipse  happens  precisely  at  the 
node*5,  they  are  not  only  total,  but  last  for  some  length  of 
time. 

When  the  sun  is  eclipsed,  the  total  darkness  is  con- 
fined to  one  particular  part  of  the  earth,  evidently  show- 
ing that  the  moon  is  smaller  than  the  earth,  since  she  can- 
not entirely  screen  it  from  the  sun.  In  fig.  1.  plate  XII. 
you  will  find  a  solar  eclipse  described  ;  S  is  the  sun,  M 
the  moon,  and  E  the  earth  ;  and  the  moon's  shadow,  you 
see,  is  not  large  enough  to  cover  the  earth.  The  lunar 
eclipses  on  the  contrary  are  visible  from  every  part  of  the 
earth,  where  the  moon  is  above  the  horizon  ;  and  we  dis- 
cover by  the  length  of  time  which  the  moon  is  in  passing 
through  the  earth's  shadow,  that  it  would  be  sufficient  to 
eclipse  her  totally,  were  she  47  times  her  actual  size  ; 
it  follows,  therefore,  that  the  earth  is  47  times  the  size  of 
the  moon. 

In  fig.  2,  S  represents  the  sun,  which  pours  forth  rays 
of  light  in  straight  lines  in  every  direction.  E  is  the 
earth,  and  M  the  moon.  Now  a  ray  of  light  coming  from 
one  extremity  of  the  sun's  disk  in  the  direction  A  B,  will 
meet  another  coming  fi-om  the  opposite  extremity  in  the 
direction  C  B  ;  the  shadow  of  the  earth  cannot  therefore 
extend  beyond  B  ;  as  the  sun  is  larger  than  the  earth,  the 
shadow  of  the  latter  is  conical,  or  the  figure  of  a  sugar 
loaf;  it  gradually  diminishes,  and  is  much  smaller  than 
the  earth  where  the  moon  passes  through  it,  and  yet  we 

525.     When  does  a  partial  eclipse  take  place  ? 526.     What 

is  the  consequence  when  an  eclipse  happens  precisely  at  the  nodes  ^ 

527.     When  the  sun  is  eclipsed  docs  the  total  darkness  extend 

to  the  whole  hemisphere  ^ 528.     What  is  shown  from  the  dark- 
ness being  confined  to  a  partioular  spot  of  the  earth  ? 529. 

By  which  figure  is  a  solar  eclipse  illustrated  t 530.     How  can 

the  comparative  size  of  the  earth  and  moon  be  determined  by  a 

lunar  eclipse  ^ 531.     How  much  larger  is  the  earth  than  the 

moon  thus  found  to  be  ? 532.     How  does  figure  2,  plate  XH. 

illustrate  this  subject  ? 


128  ON  THE  MOON. 

find  the  moon  to  be  not  only  totally  eclipsed,  but  some 
length  of  time  in  darkness,  and  hence  we  are  enabled  to 
ascertain  its  real  dimensions. 

Emily,  When  the  moon  eclipses  the  sun  to  us,  we 
must  be  eclipsed  to  the  moon  ? 

Mrs,  B,  Certainly  ;  for  if  the  moon  intercept  the 
sun's  rays,  and  cast  a  shadow  on  us,  we  must  necessarily 
disappear  to  the  moon,  but  only  partially,  as  in  fig.  1 . 

Carolinp,  There  must  be  a  great  number  of  eclipses 
in  the  distant  planets,  which  have  so  many  moons. 

Mrs,  B,  Yes,  few  days  pass  without  an  eclipse  taking 
place  ;  for  among  the  number  of  satellites,  one  or  other 
of  them  are  continually  passing  either  between  their  pla- 
net and  the  sun,  or  between  the  planet  and  each  other. 
-Astronomers  are  so  well  acquainted  with  the  motion  of 
the  planets  and  their  satellites,  that  they  have  calculated 
not  only  the  eclipses  of  our  moon,  but  those  of  Jupiter, 
with  such  perfect  accuracy,  that  it  has  afforded  a  means 
of  ascertaining  the  longitude. 

Caroline,  But  is  it  not  very  easy  to  find  both  the  lati- 
tude and  longitude  of  any  place  by  a  map  or  globe  ? 

3Irs.  B,  If  you  know  where  you  are  situated,  there 
is  no  difficulty  in  ascertaining  the  latitude  or  longitude  of 
the  place  by  referring  to  a  map  ;  but  supposing  that  you 
had  been  a  length  of  time  at  sea,  interrupted  in  your 
course  by  storms,  a  map  would  afford  you  very  little  as- 
sistance in  discovering  where  you  were. 

Caroline,  Under  such  circumstances,  I  confess  I 
should  be  equally  at  a  loss  to  discover  either  latitude  or 
longitude. 

Mrs,  B.  The  latitude  may  be  easily  found  by  taking 
the  altitude  of  the  pole  ;  that  is  to  say,  the  number  of 
degrees  that  it  is  elevated  above  the  horizon,  for  the  pole 
appears  more  elevated  as  we  approach  it,  and  less  as 
we  recede  from  it. 

Caroline,  But  unless  you  can  see  the  pole,  how  can 
you  take  its  altitude  ? 

Mrs,  B,  The  north  pole  points  constantly  towards 
one  particular  part  of  the  heavens  in  which  a  star  is  situ- 
ated,   called  the  Polar  Star  :this  star  is  visible  on  clear 

533.     When  is  the  earth  eclipsed  to  the  moon  ? 534.     Which 

figure  illustrates  the  manner  in  wliich  the  earth  is  eclipsed  to  the 

moon  ? 535.     Are  not  the  eclipses  of  the  distant  planets,  which 

have  so  many  moons,  frequent  ? 536.  What  benefit  do  we  de- 
rive from  these  eclipses  '' 


ON  THE  MOON.  l29 

nights  from  every  part  of  the  northern  hemisphere  ;  the 
altitude  of  the  polar  star  is  therefore  the  same  number 
of  degrees  as  that  of  the  pole :  the  latitude  may  also  be 
determined  by  observations  made  on  the  sun  or  any  of 
the  fixed  stars ;  the  situation  therefore  of  a  vessel  at  sea, 
with  regard  to  north  and  south,  is  easily  ascertained. 
The  difficulty  is  respecting  east  and  west,  that  is  to  say, 
its  longitude.  As  we  have  no  eastern  poles  from  which 
we  can  reckon  our  distance,  some  particular  spot  must 
be  fixed  upon  for  that  purpose.  The  English  reckon 
from  the  meridian  of  Greenwich,  where  the  royal  obser- 
vatory is  situated  ;  in  French  maps  you  will  find  that  the 
longitude  is  reckoned  from  Paris. 

The  rotation  of  the  earth  on  its  axis  in  24  hours  from 
west  to  east  occasions,  you  know,  an  apparent  motion  of 
the  sun  and  stars  in  the  contrary  direction,  and  the  sun 
appears  to  go  round  the  earth  in  the  space  of  24  hours, 
passing  over  fifteen  degrees  or  a  twenty-fourth  part  of 
the  earth's  circumference  every  hour  ;  therefore,  when 
it  is  twelve  o'clock  in  London,  it  is  one  o'clock  in  any 
place  situated  fifteen  degrees  to  the  east  of  London,  as 
the  sun  must  have  passed  the  meridian  of  that  place  an 
hour  before  he  reaches  that  of  London.  For  the  same 
reason  it  is  eleven  o'clock  to  any  place  situated  fifteen 
degrees  to  the  west  of  London,  as  the  sun  will  not  come 
to  that  meridian  till  an  hour  later. 

If  then  the  captain  of  a  vessel  at  sea  could  know  pre- 
cisely what  was  the  hour  at  London,  he  could,  by  looking 
at  his  watch,  and  comparing  it  with  the  hour  of  the  spot 
in  which  he  was,  ascertain  the  longitude. 

Emily.  But  if  he  had  not  altered  his  watch,  since  he 
sailed  from  London,  it  would  indicate  the  hour  it  was 
then  in  London. 

Mrs,  B.  True ;  but  in  order  to  know  the  hour  of  the 
day  of  the  spot  in  which  he  is,  the  captain  of  a  vessel  re- 
gulates his  watch  by  the  sun  when  it  reaches  the  meridian. 


537.     How  can  the  latitude  of  a  place  be  determined  ? 538 

Why  is  it  more  difficult  to  determine,  by  observation,  the  longitude 
than  the  latitude  of  a  place  ? 539.  From  what  place  do  the  En- 
glish reckon  longitude  ? 540.     What  does  the  rotation  of  the 

sun  upon  its  axis  in  24  hours  fi-om  west  to  east  occasion  ? 

541.  Over  how  many  degrees  does  the  sun  thus  appear  to  move 
everv  hour  ? 


J  30  ON  THE  MOON. 

Emily,  Then  if  he  had  two  watches,  he  might  keep 
one  regulated  daily,  and  leave  the  other  unaltered  ;  the 
former  would  indicate  the  hour  of  the  place  in  which  he 
was  situated,  and  the  latter  the  hour  of  London ;  and  by 
comparing  them  together,  he  would  be  able  to  calculate 
his  longitude. 

Mrs,  B,  You  have  discovered,  Emily,  a  mode  of  find- 
ing the  longitude,  which  I  have  the  pleasure  to  tell  you, 
is  universally  adopted  :  watches  of  a  superiour  construc- 
tion, called  chronometers,  or  time-keepers,  are  used  for 
this  pur|)ose  ;  but  the  best  watches  are  liable  to  imperfec- 
tions, and  should  the  time-keeper  go  too  fast,  or  too  slow, 
there  would  be  no  means  of  ascertaining  the  errour ;  im- 
plicit reliance  cannot  consequently  be  placed  upon  them. 
Recourse  is  therefore  had  to  the  eclipses  of  Jupiter's 
satellites.  A  table  is  made  of  the  precise  time  at  which 
the  several  moons  are  eclipsed  to  a  spectator  at  London; 
when  they  app*^'ar  eclipsed  to  a  spectator  in  any  other 
spot,  he  may,  by  consulting  the  table,  know  what  is  the 
hour  at  London  ;  for  the  eclipse  is  visible  at  the  same 
moment  from  whatever  place  on  the  earth  it  is  seen. 
He  has  then  only  to  look  at  the  watch  which  points  out 
the  hour  of  the  place  in  which  he  is,  and  by  observing  the 
difference  of  time  there,  and  at  London,  he  may  immedi- 
ately determine  his  longitude. 

Let  us  suppose,  that  a  certain  moon  of  Jupiter  is  al- 
ways eclipsed  at  six  o'clock  in  the  evening  ;  and  that  a 
man  at  sea  consults  his  watch,  and  finds  that  it  is  ten 
o'clock,  at  night,  where  he  is  situated,  at  the  moment  the 
eclipse  takes  place  ;  what  will  be  his  longitude  7 

Emily,  That  is  four  hours  later  than  in  London :  four 
times  fifteen  degrees  makes  60  ;  he  would,  therefore,  be 
sixty  degrees  east  of  London,  for  the  sun  must  have  pass- 
-ed  his  meridian  before  it  reaches  that  of  London. 

Mrs.  B,  For  this  reason  the  hour  is  always  later  than 
in  London,  when  the  place  is  east  longitude,  and  earlier 
when  it  is  west  longitude.     Thus  the  longitude  can  be 

542.     How  can  longitude  be  determined  at  sea  by  the  use  of  two 

watches  ? 543.     What  is  the  difficulty  in  depending  at  sea  on 

this  mode  of  finding  the  longitude  .'' 544.     How  can  longitude 

bo  determined  from  the  ecHpses  of  Jupiter's  satelhtes  ? 545. 

What  case  is  supposed  to  illustrate  this  method  of  finding  the 

longitude  of  a  place  ? 546.     How  is  east  known  from  ^s^test 

longitude  when  thus  found  ^ 


0N  THE  MOON.  131 

ascertained  whenever  the  eclipses  of  Jupiter's  moons  are 
visible. 

But  it  is  not  only  the  secondary  planets  which  produce 
eclipses,  for  the  primary  planets  near  the  sun  eclipse  him 
to  those  at  a  greater  distance  when  they  come  in  conjunc- 
tion in  the  nodes  of  their  orbits  ;  but  as  the  primary 
planets  are  much  longer  in  performing  their  course  round 
the  sun,  than  the  satellites  in  going  round  their  primary 
planets,  these  eclipses  very  seldom  occur.  Mercury  and 
Venus  have  however  passed  in  a  right  line  between  us 
and  the  sun,  but  being  at  so  great  a  distance  from  us, 
their  shadows  did  not  extend  so  far  as  the  earth ;  no 
darkness  was  therefore  produced  on  any  part  of  our  globe ; 
but  the  planet  appeared  like  a  small  black  spot,  passing 
across  the  sun's  disk  ;  this  is  called  a  transit  of  the  planet. 

It  was  by  the  last  transit  of  Venus,  that  astronomers 
were  enabled  to  calculate  with  some  degree  of  accuracy 
the  distance  of  the  earth  from  the  sun,  and  the  dimensions 
of  the  latter. 

Emily.  I  have  heard  that  the  tides  are  effected  by  the 
moon,  but  I  cannot  conceive  what  influence  it  can  have 
on  them. 

Mrs,  B,  They  are  produced  by  the  moon's  attraction 
which  draws  up  the  waters  in  a  protuberance. 

Caroline,  Does  attraction  act  on  water  more  power- 
fully than  on  land  1.  I  should  have  thought  it  would  have 
been  just  the  contrary,  for  land  is  certainly  a  more  dense 
body  than  water  1 

Mrs.  B.  Tides  do  not  arise  from  water  being  more 
strongly  attracted  than  land,  for  this  certainly  is  not  the 
case ;  but  the  cohesion  of  fluids  being  much  less  than 
that  of  solid  bodies,  they  more  easily  yield  to  the  power 
of  gravity,  in  consequence  of  which  the  waters  immedi- 
ately below  the  moon  are  drawn  up  by  it  in  a  protube- 
rance, producing  a  full  tide,  or  what  is  commonly  called 
high  water,  at  the  spot  where  it  happens.  So  far  the 
theory  of  the  tides  is  not  diflScult  to  be  understood. 

547.     Why  do  the  distant  primary  planets  eclipse  the  sun  less 

frequently  than  do  their  satellites  ? 548.     What  is  meant  by 

the  transit  of  a  planet  ? 549.     What  use  was  made  by  astrono- 
mers of  the  transit  of  Venus  ? 550.     What  occasions  the  tides  ? 

551.     How  can  the  tides  be  occasioned  by  the  attraction  of 

the  moon,  unless  water  is  acted  on  more  powerfully  by  gravita- 
tion than  the  land  - 


13^  ON  THE  MOON. 

Caroline,  On  the  contrary,  nothing  can  be  more  sim- 
ple ;  the  waters,  in  order  to  rise  up  under  the  moon,  must 
draw  the  waters  from  the  opposite  side  of  the  globe,  and 
occasion  ebb-tide,  or  low  water  in  those  parts. 

Mrs,  B,  You  draw  your  conclusion  rather  too  hastily, 
my  dear ;  for  according  to  your  theory,  we  should  have 
full  tide  only  once  in  twenty-four  hours,  that  is,  every 
time  that  we  were  below  the  moon,  while  we  find  that  we 
have  two  tides  in  the  course  of  twenty-four  hours,  and 
that  it  is  high  water  with  us  and  with  our  antipodes  at 
the  same  time. 

Caroline,  Yet  it  must  be  impossible  for  the  moon  to 
attract  the  sea  in  opposite  parts  of  the  globe,  and  in  oppo- 
site directions  at  the  same  time. 

3£rs,  B,  This  opposite  tide  is  rather  more  difficult  to 
explain,  than  that  which  is  drawn  up  beneath  the  moon  ; 
with  a  little  attention,  however,  I  hope  I  shall  be  able  to 
make  you  understand  it. 

You  recollect  that  the  earth  and  moon  are  mutually  at- 
tracted towards  a  point,  their  common  centre  of  gravity 
and  of  motion  ;  can  you  tell  me  what  it  is  that  prevents 
their  meeting  and  uniting  at  this  point  ? 

Emily,  Their  projectile  force,  which  gives  them  a 
tendency  to  fly  from  this  centre. 

Mrs,  B,  And  is  hence  called  their  centrifugal  force. 
Now  w^e  know  that  the  centrifugal  force  increases  in  pro- 
portion to  the  distance  from  the  centre  of  motion. 

Caroline,  Yes,  I  recollect  your  explaining  that  to  us, 
and  illustrating  it  by  the  motion  of  the  flyers  of  a  wind- 
mill, and  the  spinning  of  a  top. 

Emily,  And  it  was  but  the  other  day  you  showed  us 
that  bodies  weighed  less  at  the  equator,  than  in  the  polar  re- 
gions, in  consequence  of  the  increased  centrifugal  force 
in  the  equatorial  parts. 

Mrs,  B,  Very  well.  The  power  of  attraction,  on 
the  contrary,  increases  as  the  distance  from  the  centre  of 
gravity  diminishes  ;  when,  therefore,  the  two  centres  of 
gravity  and  of  motion  are  in  the  same  spot,  as  is  the  case 
with  regard  to  the  moon  and  the  earth,  the  centrifugal 

552.  How  often  do  we  have  a  high  tide  ? 553.  What  pre- 
vents the  earth  and  moon  from  bein^  drawn  together  in  their  com- 
mon centre  of  gravity  ? 554.     In   what  proportion  does  the 

centrifugal  force  increase  ? 555.     In  what  ratio  does  the  pow- 
er of  attraction  increase  ? 
11 


ON  THE  MOON.  133 

power  and  those  of  attraction,  will  be  in  inverse  propor- 
tion to  each  other  ;  that  is  to  say,  where  the  one  is  strong- 
est, the  other  will  be  weakest. 

Emily.  Those  parts  of  the  ocean,  then,  which  are 
most  strongly  attracted,  will  have  least  centrifugal  force ; 
and  those  parts  which  are  least  attracted,  will  have  the 
greatest  centrifugal  force. 

Mrs.  B.  In  order  to  render  the  question  more  simple, 
let  us  suppose  the  earth  to  be  every  where  covered  by  the 
ocean,  as  represented  in  (ng.  3,  pi.  XII.)  M  is  the  moon, 
A  B  CD  the  earth,  and  X  the  common  centre  of  gravity 
and  of  motion  of  these  two  planets.  Now  the  waters  on 
the  surface  of  the  earth,  about  A,  being  more  strongly 
attracted  than  any  other  part,  will  be  elevated  ;  the  at- 
traction of  the  moon  at  B  and  C  being  less,  and  at  D  least 
of  all.  But  the  centrifugal  force  at  D,  will  be  greatest, 
and  the  waters  there,  will  in  consequence  have  the  great- 
est tendency  to  recede  from  the  moon  ;  the  waters  at  B 
and  C  will  have  less  tendency  to  recede,  and  at  A  least 
of  all.  The  waters,  therefore,  at  D,  will  recede  furthest, 
at  the  same  time  that  they  are  least  attracted,  and  in  con- 
sequence will  be  elevated  in  a  protuberance  similar  to  that 
at  A. 

Emily.  The  tide  A,  then,  is  produced  by  the  moon's 
attraction,  and  increased  by  the  feebleness  of  the  centri- 
fugal power  in  those  parts ;  and  the  tide  D  is  produced 
by  the  centrifugal  force,  and  increased  by  th^  feebleness 
of  the  moon's  attraction  in  those  parts.* 


*  The  opinion  of  Mrs.  Bryan  concerni^^  the  tide  on  that  part  of 
the  earth  furthest  from  the  moon  is  p  »t  universally,  and  it  is  be- 
lieved, not  generally  adopted  by  writers  on  this  subject.  The 
theory  may  be  an  ingenious  one  ?  but,  it  seems  more  probable, 
that  the  centrifugal  motion  of  ^^i^Q  earth  is  only  an  auxiliary  and 
not  a  principal  cause  of  this  tide  ;  and  that  its  principal  cause  is 
the  moon's  attraction.  For  if  the  globe  were  one  solid  mciss  of 
matter,  every  part  of  it  woald  be  drawn  alike  towards  the  mooji  ; 


5?>6.     How  would  you  explain  the  production  of  the  tides  by 

Figure  3,  plate  XII  ? 557.     Is  the  opinion  of  Mrs.  Bryan  conr 

cernincr  the  tides,  universally  adopted  9 558.     What  is  thought 

a  more  probable  cause  of  the  tide  upon  the  part  of  the  earth  furthest 
from  the  moon  than  the  centrifugal  motion  of  the  earth  ? 

12 


184  ON  THE  MOON. 

Caroline,  And  when  it  is  high  water  at  A  and  D,  it  is 
low  water  at  B  and  C :  now  I  think  I  comprehend  the 
nature  of  the  tides  again,  though  I  confess  it  is  not  quite 
so  easy  as  I  at  first  thought. 

But,  Mrs.  B.,  why  does  not  the  sun  produce  tides  as 
well  as  the  moon  ;  for  its  attraction  is  greater  than  that  of 
the  moon  ? 

Mrs,  B.  It  would  be,  at  an  equal  distance,  but  our 
vicinity  to  the  moon  makes  her  influence  more  powerful. 
The  sun  has,  however,  a  considerable  effect  on  the  tides, 
and  increases  or  diminishes  them  as  it  acts  in  conjunction 
with,  or  in  opposition  to  the  moon. 

Emily,     I  do  not  quite  understand  that. 

Mrs,  B,  The  moon  is  a  month  in  going  round  the 
earth ;  twice  during  that  time,  therefore,  at  full  and  at 
change,  she  is  in  the  same  direction  as  the  sun,  both  then 
act  in  conjunction  on  the  earth,  and  produce  very  great 
tides,  called  spring  tides,  as  described  in  fig.  4.  at  A  and 
B  ;  but  when  the  moon  is  at  the  intermediate  parts  of  her 
orbit,  the  sun,  instead  of  affording  assistance,  weakens 
her  power  by  acting  in  opposition  to  it ;  and  smaller  tides 
are  produced,  called  neap  tides,  as  represented  in  i\g,  5.* 


but  as  tV^re  is  not  a  sufficient  degree  of  cohesive  attraction  in  the 
watery  parts  of  it  to  preserve  perfectly  its  form,  the  waters  upon 
Ihat  part  ot\t  nearest  the  moon  are  drawn  away  from  the  land, 
while  the  lan^K  which  is  supposed  to  constitute  the  central  regions 
of  the  globe,  is  <irawn  away  from  the  waters  upon  that  part  of  it 
most  distant  fromtjie  moon. 


*  Although  the  spring  and  neap  tides  are  produced  by  the  con- 
junction and  opposhion  of  \>ie  sun  and  moon,  yet  their  effects  are 
not  immediate  ;  the  highest  tides  happen  not  on  the  days  of  the 
full  and  chatige,  neither  do  tht  lowest  tides  happen  on  the  days 


658.     How  could  you  account  for  this  tide,  if  produced  by  the 

moon's  attraction  f 559.     As  the  sun  is  larger  than  the  moon, 

why  does  not  the  sun  produce  the  chief  influence  in  the  production 

of  the  tides.'' 560.     But  does  the  sun  exercise  no  influence  in 

the  production  of  the  tides  ? 561.     When  does  it  increase,  and 

when  diminish  the  tides  .^ 562.     AVhat  is   meant  by  the  sun 

ahd  moon  acting  in  conjunction  on  the  tides  ? 563.     What  are 

the  spring  tides  ^ -564.     What  are  the  tides  called  when  the 

sun  and  moon  are  in  opposition  ? -565.     How  would  you  explain 

the  spring  and  neap  tides  by  the  Figures  ^ 


ON  TliE  MOON.  135 

Emily,  1  have  often  observed  the  difference  of  these 
tides  when  I  have  been  at  the  sea  side. 

But  since  attraction  is  mutual  between  the  moon  and 
the  earth,  we  must  produce  tides  in  the  moon  ;  and  these 
must  be  more  considerable  in  proportion  as  our  planet  is 
larger.  And  yet  the  moon  does  not  appear  of  an  oval 
form. 

Mrs.  B,  You  must  recollect,  that  in  order  to  render 
the  explanation  of  the  tides  clearer,  we  suppose  the  whole 
surface  of  the  earth  to  be  covered  with  the  ocean ;  but 
that  is  not  really  the  case,  either  with  the  earth  or  the 
moon,  and  the  land  which  intersects  the  water  destroy?? 
the  regularity  of  the  effect. 

Caroline,  True ;  we  may,  however,  be  certain,  that 
whenever  it  is  high  water  the  moon  is  immediately  over 
our  heads. 

Mrs.  B,  Not  so  either  ;  for  as  a  similar  effect  is  pro- 
duced on  that  part  of  the  globe  immediately  beneath  the 
moon,  and  on  that  part  most  distant  from  it,  it  cannot  be 
over  the  heads  of  the  inhabitants  of  both  those  situations 
at  the  same  time.  Besides,  as  the  orbit  of  the  moon  is 
very  nearly  parallel  to  that  of  the  earth,  she  is  never  ver- 
tical but  to  the  inhabitants  of  the  torrid  zone  ;  in  that 
climate,  therefore,  the  tides  are  greatest,  and  they  dimi- 
nish as  you  recede  from  it  and  approach  the  poles. 

Caroline.  In  the  torrid  zone,  then,  I  hope  you  will 
grant  that  the  moon  is  immediately  over,  or  opposite  the 
spots  where  it  is  high  water  ? 

Mrs.  B.  I  cannot  even  admit  that ;  for  the  ocean  na- 
turally partaking  of  the  earth's  motion,  in  its  rotation  from 
west  to  east,  the  moon,  in  forming  a  tide,  has  to  contend 


of  quadratures.  But  on  account  of  the  continuation  of  motion,  it  is, 
some  time  after ^  the  exercise  of  the  sun  and  moon's  attraction,  in 
the  manner  supposed,  that  the  effect  of  their  forces  is  most  to  be 
seen.  So  that  the  greatest  spring  tides  commonly  happen  three 
days  after  the  new  and  full  moons  ',  and  the  least  iie.ap  tid/?s  three 
days  after  the  first  and  third  quarters. 


566.     How  much  after  the  conjunction  and  opposition  of  the  sun 

and  moon  do  the  spring  and  neap  tides  take  place  9 567.     In 

w-hat  parts  of  the  earth  are  the  tides  highest  ? 568.     Why  are 

they  highest  in  the  equatorial  regions  ^ 


136  ON  THE  MOON. 

against  the  eastern  motion  of  the  waves.  All  matter,  you 
know,  by  its  inertia,  makes  some  resistance  to  a  change 
of  state  ;  the  waters,  therefore,  do  not  readily  yield  to  the 
attraction  of  the  moon,  and  the  effect  of  her  influence  is 
not  complete  till  three  hours  after  she  has  passed  the  me- 
ridian, where  it  is  full  tide. 

Emily,  Pray  what  is  the  reason  that  the  tide  is  three 
quarters  of  an  hour  later  every  day  ] 

Mrs,  B,  Because  it  is  twenty-four  hours  and  three- 
quarters  before  the  same  meridian  on  our  globe  returns 
beneath  the  moon.  The  earth  revolves  on  its  axis  in 
about  twenty-four  hours  ;  if  the  moon  were  stationary, 
therefore,  the  same  part  of  our  globe  would,  every  twen- 
ty-four hours,  return  beneath  the  moon  ;  but  as  during 
our  daily  revolution  the  moon  advances  in  her  orbit,  the 
earth  must  make  more  than  a  complete  rotation  in  order  to 
bring  the  same  meridian  opposite  the  moon  :  we  are  three 
quarters  of  an  hour  in  overtaking  her.  The  tides,  there- 
fore, are  retarded  for  the  same  reason  that  the  moon  rises 
later  by  three  quarters^  of  an  hour  every  day.* 

We  have  now,  I  think,  concluded  the  observations  I 
had  to  make  to  you  on  the  subject  of  astronomy  ;  at  our 
next  interview,  I  shall  attempt  to  explain  to  you  the  ele- 
ments of  hydrostaticks. 

^  There  are  no  tides  in  lakes,  because  they  are  generally  so 
small  that  when  the  moon  is  vertical  she  attracts  every  part  alike ; 
and  by  rendering  all  the  waters  equally  light,  no  part  can  be  rais- 
ed higher  than  another.  The  Mediterranean  and  Baltick  seas 
have  very  small  elevations,  because  the  inlets  by  which  they  com- 
municate with  the  ocean  are  so  narrow,  that  they  cannot  in  so 
short  a  time  either  receive  or  discharge  enough,  sensibly  to  raise 
or  sink  their  surfaces  ? 

569.  Why  is  it  not  high  water  at  a  place,  when  the  moon  is  di- 
rectly over  the  meridian  of  it  .'* 570.     How  long  after  the  moon 

passes  the  meridian  of  a  place  before  the  effect  of  her  influence 

becomes  complete  ? 571.     Why  are  the  tides  three  quarters  of 

an  hour  later  every  day  ? -572.     Why  are  there  no  tides  on  the 

lakes  ? 573.     Why  are  the  tides  small  in  the  Mediterranean. 

and  Boiltick  seas  ?     ' 


ON  THE  MECHANICAL  PROPERTIES  OP  FLUIDS.        VA7 

CONVERSATION  X. 

ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 

Definition  of  a  Fluid ;  Distinction  between  Fluids  and 
Liquids;  Of  Non-Elasiich  Fluids  ;  Scarcely  suscepti- 
ble of  Compression  ;  Of  the  Cohesion  of  Fluids  ;  Of 
their  Gravitation ;  Of  their  Equilibrium ;  Of  their 
Pressure;  Of  Specific^  Gravity ;  Of  the  SpecificJc 
Gravity  of  Bodies  heavier  than  Water  ;  Of  those  of 
the  same  Weight  as  Water  ;  Of  those  lighter  than  Wa- 
ter;   Of  the  Specifich  Gravity  of  Fluids, 


We  have  hitherto  confined  our  attention  to  the  me- 
chanical properties  of  solid  bodies,  which  have  been  illus- 
trated, and,  I  hope,  thoroughly  impressed  upon  your  me- 
mory, by  the  conversations  we  have  subsequently  had  on 
astronomy.  It  v/ill  now  be  necessary  for  me  to  give  you 
some  account  of  the  mechanical  properties  of  fluids — a 
science  which  is  called  hydrostaticks.  A  fluid  is  a  sub- 
stance which  yields  to  the  slightest  pressure.  If  you  dip 
your  hand  into  a  basin  of  water,  you  are  scarcely  sensible 
of  meeting  with  any  resistance. 

Emily,  The  attraction  of  cohesion  is,  then,  I  suppose, 
less  powerful  in  fluids  than  in  solids  ? 

Mrs.  B.  Yes;  fluids,  generally  speaking,  are  bodies 
of  less  density  than  solids.  From  the  slight  cohesion  of 
the  particles  of  fluids,  and  the  facility  with  which  they 
slide  over  each  other,  it  is  inferred,  that  they  must  he 
small,  smooth,  and  globular  ;  smooth,  because  there  ap- 
pears to  be  little  or  no  friction  among  them  ;  and  globu- 
lar, because  touching  each  other  but  by  a  point  would  ac- 
count for  the  slightness  of  their  cohesion.* 

*  If  the  particles  of  fluids  ^e  round,  there  must  be  vacant  spaces 
between  them,  in  the  same  manner  as  there  are  vacuities  between 
cannon  balls  that  are  piled  together  ;  between  these  balls  smaller 

574.     What  is  the  science  called  that  treats   of  the  mechanical 

properties  of  fluids  ? 575.     What  is  meant  by  a  fluid  ? 576. 

In  which  is  the  attraction  of  cohesion  the  most  powerful,  solids  or 

fluids  ? 577.     What  is  inferred  from  the  slight  cohesion  of  the 

particles  of  fluids,  and  the  facility  with  which  they  slide  over  each 
other  ? 

12* 


I3§        ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS* 

Caroline.  Pray  what  is  the  distinction  between  a  fluid 
and  a  liquid  ? 

Mrs.  B.  Liquids  comprehend  only  one  class  of  fluids. 
There  is  another  class  distinguished  by  the  name  of  elas- 
tick  fluids,  or  gases,  which  comprehends  the  air  of  the 
atmosphere,  and  all  the  various  kinds  of  air  with  which 
you  will  become  acquainted  when  you  study  chemistry. 
Their  mechanical  properties  we  shall  examine  at  our  next 
meeting,  and  confine  our  attention  this  morning  to  those 
of  liquids,  or  non-elastick  fluids. 

Water  and  liquids  in  general,  are  scarcely  susceptible 
of  being  compressed,  or  squeezed  into  a  smaller  space 
than  that  which  they  naturally  occupy.  This  is  supposed 
to  be  owing  to  the  extreme  minuteness  of  their  particles, 
which,  rather  than  submit  to  compression,  force  their 
way  through  the  pores  of  the  substance  which  confines 
them.  This  was  shown  by  a  celebrated  experiment 
made  at  Florence  many  years  ago.  A  hollow  globe  of 
gold  was  filled  with  water,  and  on  its  being  submitted  to 
great  pressure,  the  water  was  seen  to  exude  through  the 
pores  of  the  gold,  which  it  covered  with  a  fine  dew. 
Fluids  gravitate  in  a  more  perfect  manner  than  solid 
bodies  ;  for  the  strong  cohesive  attraction  of  the  particles 
of  the  latter  in  some  measure  counteracts  the  eflects  of 
gravity.  In  this  table,  for  instance,  the  cohesion  of  the 
particles  of  wood  enables  four  slender  legs  to  support  a 
considerable  weight.  Were  the  cohesion  destroyed,  or, 
in  other  words,  the  wood  converted  into  a  fluid,  no  sup- 
port could  he  afforded  by  the  legs,  for  the  particles  no 


shot  may  be  plac€d,and  between  these,  other  still  smaller,  or  gravel, 
or  sand,  may  be  diffused.  In  a  similar  manner,  a  certain  quantity 
of  particles  of  sugar  can  betaken  up  in  water  without  increasing 
its  bulk,  and  when  the  water  has  dissolved  the  sugar,  salt  may  be 
dissolved  in  it,  and  yet  tiie  bulk  remain  the  same  :  and  admitting 
that  the  particles  of  water  are  round,  this  is  easily  accounted  for. 

578.      What  reason  is  given  in  the  note  for  supposing  that  the 

particles  of  fluids  are  round  ? 579.     What  is  the  distinction 

between  a  liquid  and  a  fluid  .'' 580.    Are  water  and  other  liquids 

susceptible  of  compression  .'' 581.  What  is  the  reason  for  sup- 
posing they  are  not  ? 582.     What  experiment  has  been  made 

to  show  that  liquids  are  not  compressible  ^ 583.  How  do  flu- 
ids gravitate  compared  with  solids  ? 584.     What  example  is 

given  to  show  that  solids  gravitate  in  a  less  perfect  manner  than- 
liquids  - 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS.        139 

loDger  cohering  together,  each  would  press  separately 
and  independently,  and  would  be  brought  to  a  level  with 
the  surface  of  the  earth. 

Emily.  This  want  of  cohesion  is  then  the  reason  why 
fluids  can  never  be  formed  into  figures,  or  maintained  in 
heaps  ;  for  though  it  is  true  the  wind  raises  water  into 
waves,  they  are  immediately  afterwards  destroyed  by  gra- 
vity, and  water  always  finds  its  level. 

Mrs,  B,  Do  you  understand  what  is  meant  by  the 
level,  or  equilibrium  of  fluids  ? 

Emily,  I  believe  I  do,  though  I  feel  rather  at  a  loss 
to  explain  it.  Is  not  a  fluid  level  when  its  surface  is 
smooth  and  flat,  as  is  the  case  with  all  fluids  when  in  a 
state  of  rest. 

Mrs.  B.  Smooth,  if  you  please,  but  not  flat  ;  for  the 
definition  of  the  equilibrium  of  a  fluid  is,  that  every  part 
of  the  surface  is  equally  distant  from  the  point  to  which 
gravity  tends,  that  is  to  say,  from  the  centre  of  the  earth ; 
hence  the  surface  of  all  fluids  must  be  bulging,  not  flat, 
since  they  will  partake  of  the  spherical  form  of  the  globe. 
This  is  very  evident  in  large  bodies  of  water,  such  as  the 
ocean,  but  the  sphericity  of  small  bodies  of  water  is  so 
trifling,  that  their  surfaces  appear  flat. 

This  level,  or  equilibrium  of  fluids  is  the  natural  re- 
sult of  their  particles  gravitating  independently  of  each 
other  ;  for  when  any  particle  of  a  fluid  accidentally  finds 
itself  elevated  above  the  rest,  it  is  attracted  down  to  the 
level  of  the  surface  of  the  fluid,  and  the  readiness  with 
which  fluids  yield  to  the  slightest  impression  will  enable 
the  particle  by  its  weight  to  penetrate  the  surface  of  the 
fluid  and  mix  with  it. 

Caroline.  But  I  have  seen  a  drop  of  oil  float  on  the 
surface  of  water  without  mixing  with  it. 

Mrs,  B.  That  is  because  oil  is  a  lighter  liquid  than 
water.  If  youjwere  to  pour  water  over  it,  the  oil  would 
rise  to  the  surface,  being  forced  up  by  the  superiour  gravi- 
ty of  the  water.  Here  is  an  instrument  called  a  water- 
level,  (fig.  1,  plate  XIII.)  which  is  constructed  upon  the 
principle  of  the  equilibrium  of  fluids.     It  consists  of  a 

5S5.  Why  cannot  liquids  be  moulded  into  figures  like  solids  ? 
586.   What  is  meant  by  the  level  or  equilibrium  of  fluids  ? 

587.  Of  what  is  the  level  or  equilibrium  of  fluids  the  result  ? — —* 

588.  Why  will  oil  remain  upon  the  top  of  water  ? 589.     How 

is  a  water-level  constructed  ^ 


140  ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 

short  tube,  A  B,  closed  at  both  ends,  and  containing  a 
little  water  ;  when  the  tube  is  not  perfectly  horizontal  the 
water  runs  to  the  lower  end,  and  it  is  by  this  means  that 
the  level  of  any  situation  to  which  we  apply  the  instru- 
ment, is  ascertained. 

Solid  bodies  you  may,  therefore,  consider  as  gravitat- 
ing in  masses,  for  the  strong  cohesion  of  their  particles 
makes  them  weigh  altogether,  while  every  particle  of  a 
fluid  may  be  considered  as  composing  a  separate  mass, 
gravitating  independently  of  each  other.  Hence  the  re- 
sistance of  a  fluid  is  considerably  less  than  that  of  a  solid 
body  ;  for  the  resistance  of  the  particles  acting  separate- 
ly, they  are  more  easily  overcome. 

Emily,  A  body  of  water,  in  falling,  does  certainly  less 
injury  than  a  solid  body  of  the  same  weight. 

Mrs.  B.  The  particles  of  fluids  acting  thus  indepen- 
dently, press  against  each  other  in  every  direction,  not 
only  downwards  but  upwards,  and  laterally  or  sideways; 
and  in  consequence  of  this  equality  of  pressure,  every 
particle  remains  at  rest  in  the  fluid.  If  you  agitate  the 
fluid  you  disturb  this  equality  of  pressure,  and  the  fluid 
will  not  rest  till  its  equilibrum  is  restored. 

Caroline,  The  pressure  downwards  is  very  natural  ; 
it  is  the  effect  of  gravity,  one  particle  weighing  upon 
another  presses  on  it ;  but  the  pressure  sideways,  and 
particularly  the  pressure  upwards,  I  cannot  understand. 

3Irs,  B.  If  there  were  no  lateral  pressure,  water 
would  not  run  out  of  an  opening  on  the  side  of  a  vessel. 

If  you  fill  a  vessel  with  sand,  it  will  not  run  out  of  such 
an  opening,  because  there  is  scarcely  any  lateral  pressure 
among  its  particles. 

Emily,  When  water  runs  out  of  the  side  of  a  vessel, 
is  it  not  owing  to  the  weight  of  the  water  above  the 
opening  1 

Mrs,  B,  If  the  particles  of  fluids  were  arranged  in 
regular  columns  thus,  (fig.  2.)  there  would  be  no  lateral 
pressure,  for  when  one  particle  is  perpendicularly  above 
the  other,  it  can  only  press  it  downwards  ;  but  as  it  must 
continually  happen,  that  a  particle  presses  between  two 
particles  beneath,  {{\g,  3.)  these  last  must  suffer  a  lateral 
pressure. 

590.     Why  do  solid  bodies  gravitate  in  masses  ? 591 .     Why 

is  the  resistance  of  fluids  less  than  that  of  solids  ? 592.     Why 

are  fluids  inclined  to  a  state  of  rest  or  Qquihbrium  ? 593.     Why 

will  liquids  run  out  of  an  opening  in  the  vessel  containing  them.-^ 


ON  THE  MECHANICAL  PROPERTIES  OP  FLUIDS.  141 

Emily »  The  same  as  when  a  wedge  is  driven  'into 
a  piece  of  wood,  and  separates  the  parts  laterally. 

Mrs,  B,  Yes.  The  lateral  pressure  proceeds,  there- 
fore, entirely  from  the  pressure  downwards,  or  the  weight 
of  the  liquid  above  ;  and  consequently  the  lower  the  ori- 
fice is  made  in  the  vessel,  the  greater  will  be  the  velocity 
of  the  water  rushing  out  of  it.  Here  is  a  vessel  of  water 
(fig.  5.)  with  three  stop  cocks  at  different  heights  ;  we 
shall  open  them,  and  you  will  see  with  what  different  de- 
grees of  velocity  the  water  issues  from  them.  Do  you  un- 
derstand this,  Caroline  ?* 

Caroline.  Oh  yes.  The  water  from  the  upper  spout 
receiving  but  a  slight  pressure,  on  account  of  its  vicinity 
to  the  surface,  flows  but  gently  ;  the  second  cock  having 
a  greater  weight  above  it,  the  water  is  forced  out  with 
greater  velocity,  whilst  the  lowest  cock,  being  near  the 
bottom  of  the  vessel,  receives  the  pressure  of  almost  the 
whole  body  of  water,  and  rushes  out  with  the  greatest 
impetuosity. 

Mrs.  B.  Very  well  ;  and  you  must  .observe,  that  as 
the  lateral  pressure  is  entirely  owing  to  the  pressure  down- 
wards, it  is  not  effected  by  the  horizontal  dimensions  of 
the  vessel,  which  contains  the  water,  but  merely  by  its 
depth  ;  for  as  every  particle  acts  independently  of  the 
rest,  it  is  only  the  column  of  particles,  immediately  above 
the  orifice,  that  can  weigh  upon  and  press  out  the  water. 

Emily.  The  breadth  and  width  of  the  vessel  then  can 
be  of  no  consequence  in  this  respect.  The  lateral  pres- 
sure on  one  side,  in  a  cubical  vessel,  is,  I  suppose,  not  so 
great  as  the  pressure  downwards. 


*  An  empty  bottle  being  corked,  and,  by  means  of  a  weight,  let 
down  a  certain  depth  into  the  sea,  it  will  be  broken,  or  the  cork 
will  be  driven  into  it  by  the  perpendicular  pressure.  But  a  bottle 
filled  with  water,  or  any  other  liquid,  may  be  let  down  to  any  depth, 
without  damage,  because  in  this  case  the  internal  pressure  is 
equal  to  the  external  ? 

694.     From  what  does  the  lateral  pressure  of  liquids  proceed  ? 

595.     How  would  you  illustrate  the   lateral  and  downward 

pressure  of  fluids  by  the  figures  ? 596.     What  fact  is  mentioned 

in  the  note  concerning  the  pressure  of  liquids  ? 597.     To  what 

is  the  velocity  of  liquids,  issuing  from  an  orifice  in  the  side  of  a 
vessel,  proportional  ? 


142         ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS* 

Mrs,  B,  No,  in  a  cubical  vessel  the  pressure  down- 
wards will  be  double  the  lateral  pressure  on  one  side  ;  for 
every  particle  at  the  bottom  of  the  vessel  is  pressed  upon 
by  a  column  of  the  whole  depth  of  the  fluid,  whilst  the 
lateral  pressure  diminishes  from  the  bottom  upwards  to 
the  surface,  where  the  particles  have  no  pressure. 

Caroline,  And  from  whence  proceeds  the  pressure  of 
fluids  upwards  ?  that  seems  to  me  the  most  unaccounta- 
ble, as  it  is  in  direct  opposition  to  gravity. 

Mrs,  B,  And  yet  it  is  a  consequence  of  their  pres- 
sure downwards.  When,  for  example,  you  pour  water  into 
a  tea-pot,  the  water  rises  in  the  spout  to  a  level  with  the 
water  in  the  pot.  The  particles  of  water  at  the  bottom 
of  the  pot  are  pressed  upon  by  the  particles  above  them  ; 
to  this  pressure  they  will  yield,  if  there  is  any  mode  of 
making  way  for  the  superiour  particles,  and  as  they  can- 
not descend,  they  will  change  their  direction  and  rise  in 
the  spout. 

Suppose  the  tea-pot  to  be  filled  with  columns  of  parti- 
cles of  water  similar  to  that  described  in  fig.  4.  the  par- 
ticle 1  at  the  bottom  will  be  pressed  laterally  by  the  par- 
ticle 2,  and  by  this  pressure  be  forced  into  the  spout 
where,  meeting  with  the  particle  3,  it  presses  it  upwards, 
and  this  pressure  will  be  continued,  from  3  to  4,  from  4 
to  5,  and  so  on  till  the  w  ater  in  the  spout  has  risea  to  a 
level  with  that  in  the  pot. 

Emily,  If  it  were  not  for  this  pressure  upwards,  forc- 
ing the  water  to  rise  in  the  spout,  the  equilibrium  of  tha 
fluid  would  be  destroyed. 

Caroline,  True  ;  but  then  a  tea-pot  is  wide  and  laag^^ 
and  the  weight  of  so  great  a  body  of  water  as  the  pot  will 
contain,  may  easily  force  up  and  support  so  small  a  quan- 
tity as  will  fill  the  spout.  But  would  the  same  effect  be 
produced  if  the  spout  and  the  pot  were  of  equal  dimen- 
sions ? 

Mrs.  B,  Undoubtedly  it  would.  You  may  even  re- 
verse the  experiment  by  pouring  water  into  the  spout,  and 
you  will  find  that  water  will  rise  in  the  pot  to  a  level 
with  that  in   the   spout ;  for    the  pressure  of  the   small 

598.     How  does  the  pressure  downwards,  in  a  cubical  vessel, 

oompare  with  the  lateral  pressure  ? 599.     Whence  proceeds 

the  pressure  of  liquids  upwards  ? 600.     How  would  you  illus- 
trate, from  the  figure,  the  upward  pressure  of  liquids  occasioned 

by  the  downward  pressure  ? 601.     What  will  be  the  effect,  in 

relation  to  this  sutjectj  if  water  is  poured  into  the  spout  '^ 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS.        143 

quantity  of  water  in  the  spout  will  force  up  and  support 
the  larger  quantity  in  the  pot.  In  the  pressure  upwards, 
as  well  as  that  laterally,  you  see  that  the  force  of  pressure 
depends  entirely  on  the  height,  and  is  quite  independent 
of  the  horizontal  dimensions  of  the  fluid. 

As  a  tea-pot  is  not  transparent,  let  us  try  the  experi- 
ment by  filling  this  large  glass  goblet  by  means  of  this  nar 
row  tube.  (fig.  6.) 

Caroline.  Look,  Emily,  as  Mrs.  B.  fills  it,  how  the 
water  rises  in  the  goblet,  to  maintain  an  equilibrium  with 
that  in  the  tube. 

Now,  Mrs.  B.,  will  you  let  me  fill  the  tube  by  pouring 
water  into  the  goblet. 

Mrs.  B.  That  is  impossible.  However,  you  may  try 
the  experiment,  and  I  doubt  not  but  that  you  will  be  able 
to  account  for  its  failure. 

Caroline.  It  is  very  singular,  that  if  so  small  a  co- 
lumn of  water  as  is  contained  in  the  tube  can  force  up  and 
support  the  whole  contents  of  the  gDblet ;  that  the  weight 
of  all  the  water  in  the  goblet  should  not  be  able  to  force 
up  the  small  quantity  required  to  fill  the  tube  : — oh,  I  see 
now  the  reason,  the  water  in  the  goblet  cannot  force  that 
in  the  tube  above  its  level  ;  and  as  the  end  of  the  tube  is 
considerably  higher  than  the  goblet,  it  can  never  be  filled 
by  pouring  water  into  the  goblet. 

Mrs.  B.  And  if  you  continue  to  pour  water  into  the 
goblet  when  it  is  full,  the  water  will  run  over  instead  of 
rising  above  the  level  in  the  tube. 

I  shall  now  explain  to  you  the  meaning  of  the  specifick 
gravity  of  bodies. 

Caroline.  What !  is  there  another  species  of  gravity 
with  which  we  are  not  yet  acquainted  ? 

Mrs.  B.  No  ;  the  specifick  gravity  of  a  body,  means 
simply  its  weight  compared  with  that  of  another  body  of 
the  same  size.  When  we  say,  that  substances,  such  as 
lead  and  stones  are  heavy,  and  that  others,  such  as  paper 
and  feathers,  are  light,  we  speak  comparatively  ;  that  is 
to  say,  that  the  first  are  heavy,  and  the  latter  light,  in 
comparison  with  the  generality  of  substances  in  nature. 
Would  you  call  wood  and  chalk  light  or  heavy  bodies  1 

602.  What  is  the  object  of  figure  6»  plate  XIII.? 003.  Wiial 

is  meant  by  the   specifick  gravity  of  bodies  .'' 604.     When  we 

say  that  such  bodies  as  lead  and  stones  are  heavy,  and  that  such 
as  paper  and  feathers  are  light,  how  do  v/e  speak  ' 


144        ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 

Caroline,  Some  kinds  of  wood  are  heavy,  certainly, 
as  oak  and  mahogany ;  others  are  hght,  as  deal  and  box. 

Emily,  I  think  1  should  call  wood  in  general  a  heavy 
body,  for  deal  and  box  are  light  only  in  comparison  to 
wood  of  a  heavier  description.  I  am  at  a  loss  to  deter- 
mine whether  chalk  should  be  ranked  as  a  heavy  or  a 
light  body  ;  I  should  be  inclined  to  say  the  former,  if  it 
were  not  that  it  is  lighter  than  most  other  minerals.  I 
perceive  that  we  have  but  vague  notions  of  light  and  heavy. 
I  wish  there  was  some  standard  of  comparison,  to  which 
we  could  refer  the  weight  of  all  other  bodies. 

Mrs.  B,  The  necessity  of  such  a  standard  has  been 
so  much  felt,  that  a  body  has  been  fixed  upon  for  this 
purpose.  What  substance  do  you  think  would  be  best 
calculated  to  answer  this  end  ? 

Caroline.  It  must  be  one  generally  known  and  easily 
obtained,  lead  or  iron  for  instance. 

Mrs.  B.  All  the  metals  expand  by  heat,  and  condense 
by  cold.  A  piece  of  lead,  let  us  say  a  cubick  inch  for  in- 
stance, would  have  less  specifick  gravity  in  summer  than 
in  winter  ;  for  it  would  be  more  dense  in  the  latter  season. 

Caroline.  But,  Mrs.  B.,  if  you  compare  the  weight  of 
equal  quantities  of  different  bodies,  they  will  all  be  alike. 
You  know  the  old  saying  that  a  pound  of  feathers  is  as 
heavy  as  a  pound  of  lead. 

Mrs.  B.  When  therefore  we  compare  the  weight  of 
different  kinds  of  bodies,  it  would  be  absurd  to  take  quan- 
tities of  equal  loeight^  we  must  take  quantities  of  equal 
hulk  ;  pints  or  quarts,  not  ounces  or  pounds. 

Caroline.  Very  true  ;  I  perplexed  myself  by  thinking 
that  quantity  referred  to  weight,  rather  than  to  measure. 
It  is  true,  it  would  be  as  absurd  to  compare  bodies  of  the 
same  size  in  order  to  ascertain  which  was  largest,  as  to 
compare  bodies  of  the  same  weight  in  order  to  discover 
which  was  heaviest. 

Mrs.  B.  In  estimating  the  specifick  gravity  of  bodies, 
therefore,  we  must  compare  equal  bulks,  and  we  shall 
find  that  their  specifick  gravity  will  be  proportional  to  their 

605.  Why  would  not  metals,  as  lead,  or  iron,  answer  for  the 
standard  to  determine  the  specifick  gravities  of  bodies  ?        606. 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS.        145 

weights.  The  body  which  has  been  adopted  as  a  stand* 
ard  of  reference  is  distilled  water.* 

Emily,  I  am  surprised  that  a  fluid  should  have  been 
chosen  for  this  purpose,  as  it  must  necessarily  be  contain- 
ed in  some  vessel,  and  the  weight  of  the  vessel  will  re- 
quire to  be  deducted. 

Mrs,  B,  In  order  to  learn  the  specifick  gravity  i)r  a 
solid  body,  it  is  not  necessary  to  put  a  certain  measure 
of  it  in  one  scale,  and  an  equal  measure  of  water  into 
the  other  scale ;  but  simply  to  weigh  the  body  under  trial 
in  water.  If  you  weigh  a  piece  of  gold  in  a  glass  of  water, 
will  not  the  gold  displace  just  as  much  water,  as  is  equal 
to  its  own  bulk  ? 

Caroline.  Certainly,  where  one  body  is,  another  can- 
not be  at  the  same  time ;  so  that  a  sufficient  quantity  of 
water  must  be  removed,  in  order  to  make  way  for  the 
gold. 

Mrs.  B,  Yes,  a  cubick  inch  of  water  to  make  room 
for  a  cubick  inch  of  gold  ;  remember  that  the  bulk  alone 
is  to  be  considered,  the  weight  has  nothing  to  do  with  the 
quantity  of  water  displaced,  for  an  inch  of  gold  does  not 


^  The  method  of  ascertaining  the  specifick  gravities  of  bodies  waa 
discovered  accidentally  by  Archimedes.  He  had  been  employed 
by  the  king  of  Syracuse  to  investigate  the  metals  of  a  golden  crown 
which  he  suspected  had  been  adulterated  by  the  workmen.  The 
philosopher  laboured  at  the  problem  in  vain,  till  going  one  day  into 
the  bath,  he  perceived  that  the  water  rose  in  the  bath  in  proportion 
to  tlie  bulk  of  his  bod}'^ ;  he  instantly  perceived  that  any  other  sub- 
stance of  equal  size  would  have  raised  the  water  just  as  much, 
though  one  of  equal  weight  and  less  bulk  could  not  have  produced 
the  same  effect.  He  then  got  two  masses,  one  of  gold  and  one  of 
silver,  each  equal  in  weight  to  the  crown,  and  having  filled  a  ves- 
sel very  accurately  with  water,  he  first  plunged  the  silver  mass  into 
it,  and  observed  the  quantity  of  water  that  flowed  over ;  he  then 
did  the  same  with  the  gold,  and  found  that  a  less  quantity  had  pass- 
ed over  than  before.  Hence  he  inferred  that,  though  of  equal 
weight,  the  bulk  of  the  silver  was  greater  than  that  of  the  gold,  and 
that  the  quantity  of  water  displaced  was,  in  each  experiment,  equal 
to  the  bulk  of  the  metal.  He  next  made  trial  with  the  crown,  and 
found  it  displaced  more  water  than  the  gold,  and  less  than  the  sil- 
ver, which  led  him  to  conclude,  that  it  was  neither  pure  gold  nor 
pure  silver. 


607.     Who  discovered  the  method  of  ascertaining  the  specifick 
gravities  of  bodies  f 608.  What  led  him  to  make  the  discovery  f 


146        ON  THE  MECHANICAL  PROPERTIES  OF  iFLUIDS. 

occupy  more  space,  and  therefore  will  not  displace  more 
water  than  an  inch  of  ivory,  or  any  other  substance  that 
will  sink  in  water. 

Well,  you  will  perhaps  be  surprised  to  hear  that  the 
gold  will  weigh  less  in  water,  than  it  did  out  of  it. 

Emilrj.     And  for  what  reason  ? 

Mrs,  B,  On  account  of  the  upward  pressure  of  the 
particles  of  water,  which  in  some  measure  supports  the 
gold,  and  by  so  doing  diminishes  its  weight.  If  the  body 
immersed  in  water  was  of  the  same  weight  as  that  fluid, 
it  would  be  wholly  supported  by  it,  just  as  the  water  which 
it  displaces  was  supported  previous  to  its  making  way  for 
the  solid  body.  If  the  body  is  heavier  than  the  water,  it 
cannot  be  wholly  supported  by  it ;  but  the  water  will  offer 
some  resistance  to  its  descent. 

Caroline,  And  the  resistance  which  water  offers  to  the 
descent  of  heavy  bodies  immersed  in  it,  (since  it  proceeds 
from  the  upward  pressure  of  the  particles  of  the  fluid,) 
must,  in  ail  cases,  I  suppose,  be  the  same. 

Mrs,  B,  Yes  ;  the  resistance  of  the  fluid  is  propor- 
tioned to  the  bulk,  and  not  to  the  weight  of  the  body  im- 
mersed in  it ;  all  bodies  of  the  same  size,  therefore,  lose 
the  same  quantity  of  their  weight  in  water.  Can  you  form 
any  idea*  what  this  loss  will  be  ? 

Emily,  I  should  think  it  would  be  equal  to  the  weight 
of  the  water  displaced  ;  for,  since  that  portion  of  the  wa- 
ter was  supported  before  the  immersion  of  the  solid  body, 
an  equal  weight  of  the  solid  body  will  be  supported. 

Mrs,  B,  You  are  perfectly  right :  a  body  weighed  in 
water  loses  just  as  much  of  its  weight,  as  is  equal  to  that 
of  the  water  it  displaces  :  so  that  if  you  were  to  put  the 
water  displaced  into  the  scale  to  which  the  body  is  sus- 
pended, it  would  restore  the  balance. 

You  must  observe,  that  when  you  weigh  a  body  in 
water,  in  order  to  ascertain  its  specifick  gravity,  you  must 
not  sink  the  basin  of  the  balance  in  the  water ;  but  either 
suspend  the  body  to  a  hook  at  the  bottom  of  the  basin, 
or  else  take  off  the  basin,  and  suspend  it  to  the  arm  of 
the  balance,  (fig.  7.)     Now  suppose  that  a  cubick  inch 

609.     Why  does  a  body  weigh  less  in  the  water  than  out  of  it  ? 

.     610.     To  what  is  the  resistance  of  water  to  a  body  immersed 

in  it  proportioned  ? 611.     How  much  does  a  body  weighed  in 

the  water  lose  of  its  weight  ? 612.     Which  figure  shows  the 

;3:wjmer  of  weighing  a  body  in  water  ^ 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 


147 


of  gold  weighed  19  ounces  out  of  water,  and  lost  one 
ounce  of  its  weight  by  being  weighed  in  water,  what  would 
be  its  specifick  gravity  ? 

Caroline,  The  cubick  inch  of  water  it  displaced  must 
weigh  that  one  ounce  ;  and  as  a  cubick  inch  of  gold 
weighs  19  ounces,  gold  is  19  times  as  heavy  as  water. 

Eituly,  I  recollect  having  seen  a  table  of  the  com- 
parative weights  of  bodies,  in  which  gold  appeared  to  me 
to  be  estimated  at  19  thousand  times  the  weight  of  water. 

Mrs.  B.  You  misunderstood  the  meaning  of  the  table. 
In  the  estimation  you  allude  to,  the  weight  of  water  was 
reckoned  at  1000.  You  must  observe,  that  the  weight 
of  a  substance,  when  not  compared  to  that  of  any  other, 
is  perfectly  arbitrary  ;  and  when  water  is  adopted  as  a 
standard,  we  may  denominate  its  weight  by  any  number 
we  please  ;  but  then  the  weight  of  all  bodies  tried  by  this 
standard  must  be  signified  by  proportional  numbers. 

Caroline.  We  may  call  the  weight  of  water  for  exam- 
ple, one,  and  then  that  of  gold  would  be  nineteen  ;  or  if 
we  choose  to  call  the  weight  of  water  1000,  that  of  gold 
would  be  19,000.  In  short,  the  specifick  gravity  means 
how  much  more  a  body  weighs  than  an  equal  bulk  of 
water. 

Mrs.  B.  It  is  rather  the  weight  of  a  body  compared 
with  that  of  water  ;  for  the  specifick  gravity  of  many 
substances  is  less  than  that  of  water.* 


*  Specifick  Gravities  of  Various  Bodies. 

Fine  gold     -  -  19,640 

Lead    -        -  -  11,325 

Fine  Silver  -  11,091 

Copper         -  -  9,000 

Iron               -  .  7,645 

Marble          -  -  2,705 

Glass    -        -  .  3,000 

Chalk  -        -  -  1,793 

Coal     -       -  .  1,250 


Mahogany 

1,063 

Milk 

1,034 

Rain  water 

1,000 

Oil           -        ^ 

,920 

Ice 

,908 

Brandy     - 

,920 

Living  men     - 

,891 

Cork 

,240 

Common  air    - 

-     :ii2 

Experiments  have  been  made  to  ascertain  the  specifick  gravity  of 
living  men,  in  order  to  know  what  weight  of  cork  fastened  to  a  per- 
son in  the  water  will  keep  him  from  sinkin*;,  on  the  supposition 
that  most  men  were  specifically  heavier  than  river  water  ;  but,  con- 
trary to  expectation,  it  was  found  from  trials  made  upon  ten  diffe- 


613.     What  is  the  specifick  gravity  of  livino-  men  ^ 


148        ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 

Caraline.  Then  you  cannot  ascertain  the  specifick 
gravity  of  such  substances  in  the  same  manner  as  that  of 
gold ;  for  a  body  that  is  lighter  than  water  will  float  on  its 
surface  without  displacing  any  water. 

Mrs,  B.  If  a  body  were  absolutely  light,  it  is  true 
that  it  would  not  displace  a  drop  of  water  ;  but  the  bodies 
we  are  treating  of  have  all  some  weight,  however  small ; 
and  will,  therefore,  displace  some  quantity  of  water.  If 
the  body  be  lighter  than  water,  it  will  not  sink  to  a  level 
with  the  surface  of  the  water,  and  therefore  it  will  not 
displace  so  much  water  as  is  equal  to  its  bulk  ;  but  it  will 
displace  as  much  as  is  equal  to  its  weight.  A  ship,  you 
mu:5t  have  observed,  sinks  to  some  depth  in  water,  and 
the  heavier  it  is  laden  the  deeper  it  sinks,  as  it  always 
displaces  a  quantity  of  water  equal  to  its  weight. 

Caroline,  But  you  said  just  now,  that  in  the  immer- 
sion of  gold,  the  bulk,  and  not  the  weight  of  body,  was  to 
be  considered. 

Mi's,  B,  That  is  the  case  with  all  substances  which 
are  heavier  than  water ;  but  since  those  which  are  light- 
er do  not  displace  so  much  as  their  own  bulk,  the  quan- 
tity they  displace  is  not  a  test  of  their  specifick  gravity. 

In  order  to  obtain  the  specifick  gravity  of  a  body  which 
is  lighter  than  water,  you  must  attach  to  it  a  heavy  one^ 
whose  specifick  gravity  is  known,  and  immerse  them  to- 
gether ;  the  specifick  gravity  of  the  lighter  body  may  then 
be  easily  calculated. 

Emily,  But  are  there  not  some  bodies  which  have  ex- 
actly the  same  specifick  gravity  as  water  ? 

Mrs,  B,  Undoubtedly  ;  and  such  bodies  will  remain 
at  rest  in  whatever  situation  they  are  placed  in  water. 


rent  persons,  that  their  mean  specifick  gravity  was  about  l-9th  less 
than  common  water.  So  long,  therefore,  as  the  lungs  can  be  kept 
&ee  from  water,  a  person,  although  unacquainted  with  the  art  of 
swimming,  will  not  completely  sink. 


614,  How  long  will  a  person  unacquainted  with  swimming 
remain  in  the  water  without  sinking  9 615.  How  can  the  spe- 
cifick gravity  of  bodies  lighter  than  water  be  obtained  ? 616. 

How  will  bodies  of  the  same   specifick  gravity  of  water  remain 
when  immersed  in  it  - 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS.        149 

Here  is  a  piece  of  wood,  which,  by  being  impregnated 
with  a  little  sand,  is  rendered  precisely  of  the  weight  of 
an  equal  bulk  of  water  ;  in  whatever  part  of  this  vessel  of 
water  you  place  it,  you  will  find  that  it  will  remain  sta- 
tionary. 

Caroline,  I  shall  first  put  it  at  the  bottom  ;  from 
thence,  of  course,  it  cannot  rise,  because  it  is  not  lighter 
than  water.  Now  I  shall  place  it  in  the  middle  of  the 
vessel ;  it  neither  rises  nor  sinks,  because  it  is  neither 
lighter  nor  heavier  than  the  water.  Now  I  will  lay  it  on 
the  surface  of  the  water  ;  but  there  it  sinks  a  little — 
what  is  the  reason  of  that,  Mrs.  B.  ? 

Mrs.  B,  Since  it  is  not  lighter  than  the  water,  it 
cannot  float  upon  its  surface ;  since  it  is  not  heavier  than 
water  it  cannot  sink  below  its  surface  ;  it  will  sink,  there- 
fore, only  till  the  upper  surface  of  both  bodies  are  on  a 
level,  so  that  the  piece  of  wood  is  just  covered  with  water. 
If  you  poured  a  few  drops  of  water  into  the  vessel,  (so 
gently  as  not  to  increase  their  momentum  by  giving  them 
velocity)  they  would  mix  with  the  water  at  the  surface, 
and  not  sink  lower. 

Caroline,  This  must,  no  doubt,  be  the  reason  why  in 
drawing  up  a  bucket  of  water  out  of  a  well,  the  bucket 
feels  so  much  heavier  when  it  rises  above  the  surface  of 
the  water  in  the  well  ;  for  whilst  you  raise  it  in  the  wa- 
ter, the  water  within  the  bucket  being  of  the  same  spe- 
cifick  gravity  as  the  water  on  the  outside,  will  be  wholly 
supported  by  the  upward  pressure  of  the  water  beneath 
the  bucket,  and  consequently  very  little  force  will  be  re- 
quired to  raise  it  ;  but  as  soon  as  the  bucket  rises  to  the 
surface  of  the  well  you  immediately  perceive  the  increase 
of  weight. 

Emily.  And  how  do  you  ascertain  the  specifick  gravity 
of  fluids  ? 

Mrs,  B,  By  means  of  an  instrument  called  an  hy- 
drometer, which  I  will  show  you.  It  consists  of  a  thin 
glass  ball  A,  (fig.  8,  plate  XIII.)  with  a  graduated  tube 
B,  and  the  specifick  gravity  of  the  liquid  is  estimated  by 
the  depth  to  which  the  instrument  sinks  in  it.     There  is 


617.     What  solid  body  is  of  the  same  specifick  gravity  of  water  ? 

618.     How  is  the  specifick  gravities  of  fluids  ascertained  ? 

619.     How  is  a  hydrometer  constructed  ?        620.     Which  figure 
represents  an  hydrometer  ? 

13* 


150  -OF  SPRINGS,  FOUNTAINS,  SlC* 

a  smaller  ball,  C,  attached  to  the  instrument  below,  which 
contains  a  little  mercury  ;  but  this  is  merely  for  the  pur- 
pose of  equipoising  the  instrument,  that  it  may  remain 
upright  in  the  liquid  under  trial. 

I  must  now  take  leave  of  you  ;  but  there  remain  yet 
many  observations  to  be  made  on  fluids  ;  we  shall,  there- 
fore, resume  this  subject  at  our  next  interview. 


CONVERSATION  XL 

OF  SPRINGS,  FOUNTAINS,  &/C. 

Of  the  Ascent  of  Vapour  and  the  Formation  of  Clouds; 
Of  the  Formation  and  Fall  of  Rain,  S^c, ;  Of  the 
Formation  of  Springs ;  Of  Rivers  and  Lakes ;  Of 
Fountains^ 

CAROLINE. 

There  is  a  question  I  am  very  desirous  of  asking  you 
respecting  fluids,  Mrs.  B.,  which  has  often  perplexed  me. 
What  is  the  reason  that  the  great  quantity  of  rain  which 
falls  upon  the  earth  and  sinks  into  it,  does  not,  in  the 
course  of  time,  injure  its  solidity  ?  The  sun  and  the  wind 
I  know,  dry  the  surface,  but  they  have  no  effect  on  the 
interiour  parts,  where  there  must  be  a  prodigious  accumu- 
lation of  moisture. 

Mrs,  B,  Do  you  not  know  that,  in  the  course  of  time, 
all  the  water  which  sinlis  into  the  ground  rises  out  of  it 
again  ?  It  is  the  same  water  which  successively  forms 
seas,  rivers,  springs,  clouds,  rain,  and  sometimes  hail, 
snow,  and  ice.  If  you  will  take  the  trouble  of  following 
it  through  these  various  changes,  you  will  understand  why 
the  earth  is  not  yet  drowned  by  the  quantity  of  water 
which  has  fallen  upon  it  since  its  creation ;  and  you  will 
even  be  convinced,  that  it  does  not  contain  a  single  drop- 
more  water  now,  than  it  did  at  that  period. 

Let  us  consider  how  the  clouds  were  originally  formed. 
When  the  first  rays  of  the  sun  warmed  the  surface  of  the 


621.  What  is  the  reason  that  the  great  quantity  of  rain  which 
falls  upon  the  earth  and  sinks  into  it,  does  not,  in  the  course  of 
time,  injure  its  solidity  ' 


OP  SPRINGS,  FOUNTAINS,  &C.  151 

earth,  the  heat,  by  separating  the  particles  of  water,  ren- 
dered them  lighter  than  the  air.  This,  you  know,  is  the 
case  with  steam  or  vapour.     What  then  ensues  ? 

Caroline.  When  lighter  than  the  air  it  will  naturally 
rise  ;  and  now  I  recollect  your  telling  us  in  a  preceding 
lesson,  that  the  heat  of  the  sun  transformed  the  particles 
of  water  into  vapour,  in  consequence  of  which  it  ascended 
into  the  atmosphere,  where  it  formed  clouds. 

Mrs.  B.  We  have  then  already  followed  water  through 
two  of  its  transformations ;  from  water  it  becomes  vapour, 
and  from  vapour  clouds. 

Emily.  But  since  this  watery  vapour  is  lighter  than 
the  air,  why  does  it  not  continue  to  rise  ?  and  why  does 
it  unite  again  to  form  clouds  ? 

Mrs.  B.  Because  the  atmosphere  diminishes  in  den- 
sity, as  it  is  more  distant  from  the  earth.  The  vapour 
therefore  which  the  sun  causes  to  exhale,  not  only  from 
seas,  rivers,  and  lakes,  but  likewise  from  the  moisture  on 
the  land,  rises  till  it  reaches  a  region  of  air  of  its  own  spe- 
cifick  gravity  ;  and  there,  you  know,  it  will  remain  sta- 
tionary. By  the  frequent  accession  of  fresh  vapour  it  gra- 
dually accumulates,  so  as  to  form  those  large  bodies  of  va- 
pour, which  we  call  clouds  ;  and  these  at  length  becoming 
too  heavy  for  the  air  to  support,  they  fall  to  the  ground. 

Caroline.  They  do  fall  to  the  ground,  certainly,  when 
it  rains ;  but  according  to  your  theory,  I  should  have  ima- 
gined, that  when  the  clouds  became  too  heavy  for  the 
region  of  air  in  which  they  were  situated  to  support  them, 
they  would  descend  till  they  reached  a  stratum  of  air  of 
their  own  weight,  and  not  fall  to  the  earth  ;  for  as  clouds 
are  formed  of  vapour,  they  cannot  be  so  heavy  as  the  low- 
est regions  of  the  atmosphere,  otherwise  the  vapour  would 
not  have  risen. 

Mrs.  B.  If  you  examine  the  manner  in  which  the 
clouds  descend,  it  will  obviate  this  objection.  In  falling, 
several  of  the  watery  particles  come  within  the  sphere  of 


602.  What  is  the  cause  of  the  ascent  of  vapour  or  steam  ? 

623.     How  are  the  clouds  formed  ? 624.     But  since  vapour  is 

lio;hter  than  the  air,  why  does  it  not  continue  to   rise  ?  and  why 

does  it  unite  again  to  form  clouds  .'' 625.     What  prevents  the 

clouds  remaining  in  the  atmosphere  where  they  are  formed  ? 

626.     Why  do  the  clouds  descend  to  the  earth  in  drops  of  water 
instead  of  vapour,  as  they  ascended  ? 


152  OF  SPRINGS,  FOUNTAINS,  &C. 

each  other's  attraction,  and  unite  in  the  form  of  a  drop  of 
water.  The  vapour,  thus  transformed  into  a  shower,  is 
heavier  than  any  part  of  the  atmosphere,  and  consequent- 
ly descends  to  the  earth. 

Caroline.     How  wonderfully  curious  ! 

Mrs,  B,  It  is  impossible  to  consider  any  part  of  na- 
ture attentively  without  being  struck  with  admiration  at 
the  wisdom  it  displays ;  and  I  hope  you  will  never  con- 
template these  wonders  without  feeling  your  heart  glow 
with  admiration  and  gratitude  towards  their  bounteous 
Author.  Observe,  that  if  the  waters  were  never  drawn 
out  of  the  earth,  all  vegetation  would  be  destroyed  by  the 
excess  of  moisture  ;  if,  on  the  other  hand,  the  plants 
were  not  nourished  and  refreshed  by  occasional  showers, 
the  drought  would  be  equally  fatal  to  them.  If  the 
clouds  constantly  remain  in  a  state  of  vapour,  they  might, 
as  you  remarked,  descend  into  a  heavier  stratum  of  the 
atmosphere,  but  could  never  fall  to  the  ground  ;  or  were 
the  power  of  attraction  more  than  sufficient  to  convert  the 
vapour  into  drops,  it  would  transform  the  cloud  into  a 
mass  of  water,  which,  instead  of  nourishing,  would  destroy 
the  produce  of  the  earth. 

Water  then  ascends  in  the  form  of  vapour,  and  descends 
in  that  of  rain,  snow,  or  hail,  all  of  which  ultimately  be- 
come water.  Some  of  this  falls  into  the  various  bodies 
of  water  on  the  surface  of  the  globe,  the  remainder  upon 
the  land.  Of  the  latter,  part  re-ascends  in  the  form  of 
vapour,  part  is  absorbed  by  the  roots  of  vegetables  and 
part  descends  into  the  bowels  of  the  earth,  where  it  for^ 
springs. 

Emily,     Is  rain  and  spring-water  then  the  same  ? 

Mrs,  B.  Yes,  originally.  The  only  difference  be- 
tween rain  and  spring  water,  consists  in  the  foreign  par- 
ticles which  the  latter  meets  with  and  dissolves  in  its  pas- 
sage through  the  various  soils  it  traverses. 

Caroline.  Yet  spring  water  is  more  pleasant  to  the 
taste,  appears  more  transparent,  and,  I  should  have  sup- 
posed, would  have  been  more  pure  than  rain  water. 

My^s.  B.  No  ;  excepting  distilled  water,  rain  water  is 
the  most  pure  we  can  obtain  ;  and  it  is  its  purity  which 
renders  it  insipid,  whilst  the  various  salts  and  different 

627.     What  are  the  several  changres  which  water  undergoes  in 

its  ascent  and  descent  ? 628.     What  is  the  difference  between 

rain  and  spring  water  ^ 629,     Which  is  the  most  pure  ? 


OP  SPRINGS,  FOUNTAINS,  &C.  153 

ingredients,  dissolved  in  spring  water,  give  it  a  species  of 
flavour,  without  in  any  degree  affecting  its  transparency  ; 
and  the  fihration  it  undergoes  through  gravel  and  sand  in 
the  bowels  of  the  earth,  cleanses  it  from  all  foreign  matter 
which  it  has  not  the  power  of  dissolving. 

When  rain  falls  on  the  surface  of  the  earth,  it  continues 
making  its  way  downwards  through  the  pores  and  cre- 
vices in  the  ground.  When  several  drops  meet  in  their 
subterraneous  passage,  they  unite  and  form  a  little  rivulet ; 
this,  in  its  progress,  meets  with  other  rivulets  of  a  similar 
description,  and  they  pursue  their  course  together  in  the 
bowels  of  the  earth,  till  they  are  stopped  by  some  sub- 
stance which  they  cannot  penetrate. 

Caroline*  But  you  said  that  water  could  penetrate  even 
the  pores  of  gold,  and  they  cannot  meet  with  a  substance 
more  dense  ?  ^ 

Mrs,  B.  But  water  penetrates  the  pores  of  gold  only 
when  under  a  strong  compressive  force,  as  in  the  Floren- 
tine experiment ;  now  in  its  passage  towards  the  centre 
of  the  earth,  it  is  acted  upon  by  no  other  power  than  gra- 
vity, which  is  not  sufficient  to  make  it  force  its  way  even 
through  a  stratum  of  clay.  This  species  of  earth,  thougb 
not  remarkably  dense,  being  of  great  tenacity,  will  not 
admit  the  particles  of  water  to  pass.  When  water  en- 
counters any  substance  of  this  nature,  therefore,  its  pro- 
gress is  stopped,  and  the  pressure  of  the  accumulating 
waters  forms  a  bed,  or  reservoir.  This  will  be  more  clear- 
ly explained  by  fig.  9,  plate  XIII.  which  represents  a  sec- 
tion, or  the  interiour  of  a  hill  or  mountain.  A  is  a  body 
of  water  such  as  I  have  described,  which  when  filled  up 
as  high  as  B  (by  the  continual  accession  of  water  it  re- 
ceives from  the  ducts  or  rivulets  a,  «,  «,  a,)  finds  a  pas- 
sage out  of  the  cavity,  and,  impelled  by  gravity,  it  runs 
on,  till  it  makes  its  way  out  of  the  ground  at  the  side  of 
the  hill,  and  there  forms  a  spring,  C. 

Caroline.  Gravity  impels  downward  towards  the  cen- 
tre of  the  earth  ;  and  the  spring  in  this  figure  runs  in  a 
horizontal  direction. 

630.     What  renders  spring  water  more  pleasant  to  the  taste,  if 

it  is  less  pure  than  rain  water  ? 631.     How  are  springs  and  ri-* 

vulets  at  first  formed  ? 632.     Through  what  species  of  earth 

will  not  water  pass  ? 633.    Which  figure  represents  the  manner 

in  which  springs  are  formed  ? ^634.     How  would  you  explain 

this  figure  ' 


154  OF  SPRINGS,  FOUNTAINS,  &C. 

Mrs,  B.  Not  entirely.  There  is  some  declivity  from 
the  reservoir  to  the  spot  where  the  water  isues  out  of 
the  ground  ;  and  gravity,  you  know,  will  bring  bodies 
down  an  inclined  plane,  as  well  as  in  a  perpendicular  di- 
rection. 

Caroline,  But  though  the  spring  may  descend  on  first 
issuing,  it  must  afterward  rise  to  reach  the  surface  of  the 
earth  ;  and  that  is  in  direct  opposition  to  gravity. 

3Irs,  B,  A  spring  can  never  rise  above  the  level  of  the 
reservoir  whence  it  issues  ;  it  must,  therefore,  find  a  pas- 
sage to  some  part  of  the  surface  of  the  earth  that  is  lower 
or  nearer  the  centre  than  the  reservoir.  It  is  true  that, 
in  this  figure,  the  spring  rises  in  its  passage  from  B  to  C 
occasionally  ;  but  this,  I  think,  with  a  little  reflection,  you 
will  be  able  to  account  for. 

Ernlly,  Oh  yes ;  it  is  owing  to  the  pressure  of  fluids  up- 
wards, and  the  water  rises  in  the  duct  upon  the  same  prin- 
ciple as  it  rises  in  the  spout  of  a  tea-pot ;  that  is  to  say, 
in  order  to  preserve  an  equilibrium  with  the  water  in  the 
reservoir.  Now  I  think  I  understand  the  nature  of 
springs ;  the  water  will  flow  through  a  duct,  whether  as- 
cending or  descending,  provided  it  never  rises  higher  than 
the  reservoir. 

Mrs,  B,  Water  may  thus  be  conveyed  to  every  part 
of  a  town,  and  to  the  upper  part  of  the  houses,  if  it  is  ori- 
ginally brought  from  a  height  superiour  to  any  to  which  it 
is  conveyed.  Have  you  never  observed,  when  the  pave- 
ment of  the  streets  have  been  mending,  the  pipes  which 
serve  as  ducts  for  the  conveyance  of  the  water  through 
the  town  1 

Emily.  Yes,  frequently  ;  and  I  have  remarked  that 
when  any  of  these  pipes  have  been  opened,  the  water 
rushes  upwards  from  them  with  great  velocity,  which  I 
suppose  proceeds  from  the  pressure  of  the  water  in  the  re- 
servoir, which  forces  it  out. 

Caroline.  I  recollect  having  once  seen  a  very  curious 
glass,  called  Tantalus's  cup ;  it  consists  of  a  goblet,  con- 
taining a  small  figure  of  a  man,  and  whatever  quantity  of 
water  you  pour  into  the  goblet,  it  never  rises  higher  than 

635.  How  high  may  a  springr  rise  ? 636.  On  what  princi- 
ple does  water  ascend  as  well  as  descend  in  its  course,  as  is  often 

the  case  in  being  carried  in  ducts  ? 637.     What  is  called  1  an« 

talus's  cup  ? 


OF  sphings,  fountains,  &c.  155 

the  breast  of  the  figure.  Do  you  know  how  that  is  con- 
trived 1 

Mrs,  B,  It  is  by  means  of  a  syphon,  or  bent  tube, 
which  is  concealed  in  the  body  of  the  figure.  It  rises 
through  one  of  the  legs,  as  high  as  the  breast,  and  there 
turning,  descends  through  the  other  leg,  and  from  thence 
through  the  foot  of  the  goblet,  w  here  the  water  runs  out. 
(fig.  I,  plate  XIV.)  When  you  pour  water  into  the  glass 
A,  it  must  rise  in  the  syphon  B,  in  proportion  as  it  rises 
in  the  glass  ;  and  when  the  glass  is  filled  to  a  level  with 
the  upper  part  of  the  syphon,  the  water  will  run  out 
through  the  other  leg  of  the  figure,  and  will  continue  run- 
ning out,  as  fast  as  you  pour  it  in  ;  therefore  the  glass 
can  never  fill  any  higher. 

Emily,  I  think  the  new  well  that  has  been  made  at 
our  country-house,  must  be  of  that  nature.  We  had  a 
great  scarcity  of  water,  and  my  father  has  been  at  con- 
siderable expense  to  dig  a  well  ;  after  penetrating  to  a 
great  depth  before  water  could  be  found,  a  spring  was  at 
length  discovered,  but  the  water  rose  only  a  few  feet  above 
the  bottom  of  the  well  ;  and  sometimes  it  is  quite  dry. 

Mrs,  B,  This  has  however,  no  analogy  to  Tantalus's 
cup,  but  is  owing  to  the  very  elevated  situation  of  your 
country  house. 

Emily,  I  believe  I  guess  the  reason.  There  cannot 
be  a  reservoir  of  water  near  the  summit  of  a  hill ;  as  in 
such  a  situation  there  will  not  be  a  sufficient  number  of 
rivulets  formed  to  supply  one  ;  and  without  a  reservoir, 
there  can  be  no  spring.  In  such  situations,  therefore,  it 
is  necessary  to  dig  very  deep,  in  order  to  meet  with  a 
spring  ;  and  when  we  give  it  vent,  it  can  rise  only  as 
high  as  the  reservoir  from  whence  it  flows,  which  will  be 
but  little,  as  the  reservoir  must  be  situated  at  some  con- 
siderable depth  below  the  summit  of  the  hill. 

Caroline,  Your  explanation  appears  very  clear  and 
satisfactory.  But  I  can  contradict  it  from  experience. 
At  the  very  top  of  a  hill,  near  our  country-house,  there 
is  a  large  pond,  and,  according  to  your  theory,  it  would 
be  impossible  there  should  be  springs  in  such  a  situation  to 
supply  it  with  water.     Then  you  know  that  I  have  crossed 

638.     By  what  means  is  the  water  prevented  from  rising  to  the 

head  of  the  figure  ? 639.     Why  must  weJIs  on  high  land  be  dug 

deep  in  order  to  be  supplied  with  water  .'' 


156  OF  Springs,  fountains,  &.c. 

the  Alps,  and  I  can  assure  you,  that  there  is  a  fine  lake 
on  the  summit  of  Mount  Cenis,  the  highest  mountain  we 
passed  over. 

Mrs.  B.  Were  there  a  lake  on  the  summit  of  Mount 
Blanc,  which  is  the  highest  of  the  Alps,  it  would  indeed 
be  wonderful.  But  that  on  Mount  Cenis  is  not  at  all 
contradictory  to  our  theory  of  springs  ;  for  this  mountain 
is  surrounded  by  others  much  more  elevated,  and  the 
springs  which  feed  the  lake  must  descend  from  reservoirs 
of  water  formed  in  those  mountains.  This  must  also  be 
the  case  with  the  pond  on  the  top  of  the  hill  ;  there  is 
doubtless  some  more  considerable  hill  in  the  neighbour- 
hood which  supplies  it  with  water. 

Emily,  1  comprehend  perfectly,  why  the  water  in  our 
well  never  rises  high  :  but  I  do  not  understand  why  it 
should  occasionally  be  dry. 

Mrs,  B,  Because  the  reservoir  from  which  it  flows 
being  in  an  elevated  situation,  is  but  scantily  supplied 
with  water  ;  after  a  long  drought,  therefore,  it  may  be 
drained,  and  the  spring  dry,  till  the  reservoir  be  reple- 
nished by  fresh  rains.  It  is  not  uncommon  to  see  springs 
flow  with  great  violence  in  wet  weather,  and  at  other 
times  be  perfectly  dry. 

Caroline.  But  there  is  a  spring  in  our  grounds  which 
more  frequently  flows  in  dry  than  in  wet  weather  :  how  is 
that  to  be  accounted  for  ? 

Mrs.  B.  The  spring  probably  comes  from  a  reservoir 
at  a  great  distance,  and  situated  very  deep  in  the  ground : 
it  is,  therefore,  some  length  of  time  before  the  rain  reaches 
the  reservoir,  and  another  considerable  portion  must 
elapse,  whilst  the  water  is  making  its  way  from  the 
reservoir  to  the  surface  of  the  earth  ;  so  that  the  dry  wea- 
ther may  probably  have  succeeded  the  rains  before  the 
spring  begins  to  flow,  and  the  reservoir  may  be  exhausted 
by  the  time  the  wet  weather  sets  in  again. 

Caroline.  I  doubt  not  but  this  is  the  case,  as  the 
spring  is  in  a  very  low  situation,  therefore  the  reservoir 
may  be  at  a  great  distance  from  it. 

Mrs,  B.  Springs,  which  do  not  constantly  flow  are 
called  intermitting,  and  are  occasioned  by  the  reservoir 

640.     How  can  the  lake  on  Mount  Cenis,  one   of  the  Alps,  be 

reconciled  to  the  theory  of  springs  which  has   been  ^iven  ? 

641.      Why  are   wells  frequently  dry  ? 642.     Why  do  some 

springs  flow   more  in  dry  than   wet  weather .'' 643.      What 

springs  are  called  intermitting  P 


OP  SPRINGS,  FOUNTAINS,  &/C.  157 

being  imperfectly  supplied.  Independently  of  the  situ- 
ation, this  is  always  the  case  when  the  duct  or  ducts 
which  convey  the  water  into  the  reservoir  are  smaller 
than  those  which  carry  it  off. 

Caroline,  If  it  run  out  faster  than  it  run  in,  it  will 
of  course  sometimes  be  empty.  And  do  not  rivers  also 
derive  their  source  from  springs  1 

Mrs,  B,  Yes,  they  generally  take  their  source  in 
mountainous  countries,  where  springs  are  most  abundant. 

Caroline,  I  understood  you  that  springs  were  more 
rare  in  elevated  situations. 

Mrs.  B,  You  do  not  consider  that  mountainous  coun- 
tries abound  equally  with  high  and  low  situations.  Re- 
servoirs of  water,  which  are  formed  in  the  bosom  of  moun- 
tains, generally  find  a  vent  either  on  their  declivity,  or  in 
the  valley  beneath  ;  while  subterraneous  reservoirs  formed 
in  a  plain,  can  seldom  find  a  passage  to  the  surface  of  the 
earth,  but  remain  concealed,  unless  discovered  by  digging 
a  well.  When  a  spring  once  issues  at  the  surface  of  the 
earth  it  continues  its  course  externally,  seeking  always  a 
lower  ground,  for  it  can  no  longer  rise. 

Emily,  Then  what  is  the  consequence,  if  the  spring, 
or  I  should  now  rather  call  it  a  rivulet,  runs  into  a  situa- 
tion which  is  surrounded  by  higher  ground  ? 

Mrs,  B,  Its  course  is  stopped,  the  water  accumulates, 
and  it  forms  a  pool,  pond,  or  lake,  according  to  the  di- 
mensions of  the  body  of  water.  The  lake  of  Geneva,  in 
all  probability,  owes  it  origin  to  the  Rhone,  which  passes 
through  it ;  if,  when  this  river  first  entered  the  valley, 
which  now  forms  the  bed  of  the  Lake,  it  found  itself  sur- 
rounded by  higher  grounds,  its  waters  would  there  accu- 
mulate, till  they  rose  to  a  level  with  that  part  of  the  valley 
where  the  Rhone  now  continues  its  course  beyond  the 
Lake,  and  from  whence  it  flows  through  valleys,  occasion- 
ally forming  other  small  lakes  till  it  reaches  the  sea. 

Emily,     And  are  not  fountains  of  the  nature  of  springs  ? 

Mrs,  B,  Exactly.  A  fountain  is  conducted  perpen- 
dicularly upwards,  by  the  spout  or  adjutage  A,  through 

644.     Why  do  rivers  usually  have  their  source  in  mountainous 

regions  ? 645.     When  a  spring  once  issues  from  the  surface  of 

the  earth  what  is  its  course  ? 646.     What  is  the  consequence 

if  a  spring  runs  into  a  situation  which  is  surrounded  by  higher 

ground  ?; 647.     How  was  lake  Geneva  probably  formed  •'— »- 

€48.    Are  artificial  fountains  of  the  nature  of  springs  ^ 
14 


158  ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

which  it  flows  ;  and  it  will  rise  nearly  as  high  as  the  reser- 
voir B,  from  whence  it  proceeds.     (Plate  XIV.  figure  2.) 

Caroline,     Why  not  quite  as  high  ? 

Mrs,  B,  Because  it  meets  with  resistance  from  the 
air  in  its  ascent ;  and  its  motion  is  impeded  by  friction 
against  the  spout,  where  it  rushes  out. 

Emilif,  But  if  the  tube  through  which  the  water  rises 
be  smooth,  can  there  be  any  friction  ?  especially  with  a 
fluid  whose  particles  yield  to  the  slightest  impression. 

Mrs,  B,  Friction  (as  we  observed  in  a  former  les- 
son,) may  be  diminished  by  polishing,  but  can  never  be 
entirely  destroyed;  and  though  fluids  are  less  susceptible 
of  friction  than  solid  bodies,  they  are  still  affected  by  it. 
Another  reason  why  a  fountain  will  not  rise  so  high  as  its 
reservoir,  is,  that  as  all  the  particles  of  water  spout  from 
the  tube  with  an  equal  velocity,  and  as  the  pressure  of  the 
air  upon  the  exteriour  particles  must  diminish  their  velo- 
city, they  will,  in  some  degree,  strike  against  the  under 
parts,  and  force  them  sidevvays,  spreading  the  column  into 
a  head,  and  rendering  it  both  wider  and  shorter  than  it 
otherwise  would  be. 

At  our  next  meeting,  we  shall  examine  the  mechanical 
properties  of  the  air,  which,  being  an  elastick  fluid,  differs 
in  many  respects  from  liquids. 

649.     Which  figure  represents  an  artificial  fountain  ? 650. 

Why  in  that  representation  does  not  the  water  rise  as  high  as  the 
reservoir  ? 


CONVERSATION  XII. 

ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

Ofihe  Spring  or  Elasticity  of  the  Air ;  Of  the  weight 
of  the  Air ;  Experiments  with  the  Air  Pump  ;  Of  the 
Barometer;  Mode  of  weighing  Air ;  Specijick  Gravity 
of  Air ;  Of  Pumps  ;  Description  of  the  Sucking 
Pump;  Description  of  the  Forcing  Pump. 

MRS.  B. 

At  our  last  meeting  we  examined  the  properties  of 
fluids  in  general,  and  more  particularly  of  such  fluids  as 
are  called  liquids. 


ON  THE  MECHANrCAL  PROPERTIES  OF  AIR.  159 

There  is  another  class  of  fluids,  distinguished  by  the 
name  of  aeriform  or  elastick  fluids,  the  principal  of  which 
is  the  air  we  breathe,  which  surrounds  the  earth,  and  is 
called  the  atmosphere. 

Emihj.  There  are  then  other  kinds  of  air,  besides  the 
atmosphere. 

3Irs,  B.  Yes  ;  a  great  variety ;  but  they  differ  only 
in  their  chemical,  and  not  in  their  mechanical  properties; 
and  as  it  is  the  latter  we  are  to  examine,  we  shall  not  at 
present  inquire  into  their  composition,  but  confine  our 
attention  to  the  mechanical  properties  of  elastick  fluids  in 
general. 

Caroline,     And  from  whence  arises  this  difference  1 

Mrs,  B,  There  is  no  attraction  of  cohesion  between 
the  particles  of  elastick  fluids  ;  so  that  the  expansive  pow- 
er of  heat  has  no  adversary  to  contend  with  but  gravity  ; 
any  increase  of  temperature,  therefore,  expands  elastick 
fluids  prodigiously,  and  a  diminution  proportionally  con- 
denses them. 

The  most  essential  point  in  which  air  differs  from  other 
fluids,  is  by  its  spring  or  elasticity  ;  that  is  to  say,  its 
power  of  increasing  or  diminishing  in  bulk,  according  as 
it  is  more  or  less  compressed  ;  a  power  of  which  I  have 
informed  you  liquids  are  almost  wholly  deprived. 

Emily.  I  think  I  understand  the  elasticity  of  the  air 
very  well  from  what  you  formerly  said  of  it ;  (see  p.  32.) 
but  what  perplexes  me  is,  its  having  gravity  ;  if  it  is  heavy 
and  we  are  surrounded  by  it,  why  do  we  not  feel  its 
weight  ? 

Caroline,  It  must  be  impossible  to  be  sensible  of  the 
weight  of  such  infinitely  small  particles,  as  those  of  which 
the  air  is  composed  :  particles  which  are  too  small  to  be 
seen,  must  be  too  light  to  be  felt. 

Mrs.  B.  You  are  mistaken,  my  dear  ;  the  air  is  much 
heavier  than  you  imagine  ;  it  is  true,  that  the  particles 
which  compose  it  are  small  ;  but  then,  reflect  on  their 
quantity :  the  atmosphere  extends  to  alaout  the  distance 

651 .     How  are  the  fluids  called  air  distinguished  from  hquids  ? 

G52.     How  do  the  other  kinds  of  air  diff'er  from  atmospherick 

air  ? 653.     Has  the  attraction  of  cohesion  any  influence  upon 

the  particles  of  elastick   fluids  ? 654.     What'effect  does  heat 

have  on  them  ? 655.     What  is   to  be  understood  by  the  elasti- 
city of  the  atmosphere  ? 656.     To  what  distance  from  the  earth 

doDs  the  atmosphere  extend  ? 


160  ON  THE  MECHANICAL  PROPERTIES  OP  AIR. 

of  45  miles  from  the  earth ;  and  its  gravity  is  such,  that 
a  man  of  middling  stature  is  computed  (when  the  air  is 
heaviest)  to  sustain  the  weight  of  about  14  tons.* 

Caroline.  Is  it  possible  !  I  should  have  thought  such 
a  weight  would  have  crushed  any  one  to  atoms. 

Mrs,  B,  That  would,  indeed,  be  the  case,  if  it  were 
not  for  the  equality  of  the  pressure  on  every  part  of  the 
body ;  but  when  thus  diffused  we  can  bear  even  a  much 
greater  weight,  without  any  considerable  inconvenience. 
In  bathing  we  support  the  weight  and  pressure  of  the  wa- 
ter, in  addition  to  that  of  the  atmosphere  ;  but  because  this 
pressure  is  equally  distributed  over  the  body,  we  are 
scarcely  sensible  of  it ;  whilst  if  your  shoulders,  your  head, 
or  any  particular  part  of  your  frame  were  loaded  with  the 
additional  weight  of  a  hundred  pounds,  you  would  soon 
sink  under  the  fatigue.  Besides  this,  our  bodies  contain 
air,  the  spring  ofw^hich  counterbalances  the  weight  of  ex- 
ternal air,  and  renders  us  less  sensible  of  its  pressure. 

Caroline,  But  if  it  were  possible  to  relieve  me  from  the 
weight  of  the  atmosphere,  should  I  not  feel  more  light 
and  agile  ? 

Mrs,  B.  On  the  contrary,  the  air  within  you,  meeting 
with  no  external  pressure  to  restrain  its  elasticity,  would 
distend  your  body,  and  at  length,  bursting  the  parts  which 
confined  it,  put  a  period  to  your  existence. 

Caroline,  This  weight  of  the  atmosphere,  then,  which 
I  was  so  apprehensive  would  crush  me,  is,  in  reality,  es- 
sential to  my  preservation. 

Emily,  I  once  saw  a  person  cupped,  and  was  told 
that  the  swelling  of  the  part  under  the  cup  was  produced 
by  taking  away  from  that  part  the  pressure  of  the  atmo- 
sphere ;  but  I  could  not  understand  how  this  pressure  pro- 
duced such  an  effect. 

Mrs,  B,  The  air  pump  affords  us  the  means  of  mak- 
ing a  great  variety  of  interesting  experiments  on  the 
weight  and  pressure  of  the  air :  some  <5f  them  you  have 


^'  The  height  to  which  the  atmosphere  extends  has  never  been 
accurately  ascertained  ;  but  at  a  greater  distance  than  45  miles  it 
ceases  to  reflect  the  sun's  rays. 

657.     What  weight  of  air  is  a  common  sized  man  supposed  to 

sustain  ? 658.     Why  does  not  such  a  weight  crush  him  to 

atoms  ? 659.     What  would  be  the  consequence,  if  the  weight 

of  external  air  were  removed  from  us  ? 


ON  THE  MECHANICAL  PROPERTIES  OP  AIR.       161 

already  seen.  Do  you  not  recollect,  that  in  a  vacuum  pro- 
duced within  the  air  pump,  substances  of  various  weights 
fell  to  the  bottom  in  the  same  time  ?  why  does  not  this 
happen  in  the  atmosphere  ? 

Caroline,  I  remember  you  told  us  it  was  owing  to  the 
resistance  which  light  bodies  meet  with  from  the  air  dur- 
ing their  fall. 

Mrs*  B.  Or,  in  other  words,  to  the  support  which 
they  received  from  the  air,  and  which  prolonged  the  time 
of  their  fall.  Now,  if  the  air  were  destitute  of  weight, 
how  could  it  support  other  bodies  or  retard  their  fall  1 

I  shall  now  snow  you  some  other  experiments,  which 
illustrate,  in  a  striking  manner,  both  the  weight  and  elas- 
ticity of  air.  I  shall  tie  a  piece  of  bladder  over  this  glass 
receiver,  which,  you  will  observe,  is  open  both  at  the  top 
as  well  as  below. 

Caroline,     Why  do  you  wet  the  bladder  first  1 

Mrs.  B,  It  expands  by  wetting,  and  contracts  in 
drying  ;  it  is  also  more  soft  and  pliable  when  wet,  so 
that  I  can  make  it  fit  better,  and  when  dry  it  will  be 
tighter.  We  must  hold  it  to  the  fire  in  order  to  dry  ; 
but  not  too  near,  lest  it  should  burst  by  sudden  contrac- 
tion. Let  us  now  fix  it  on  the  air-pump  and  exhaust  the 
air  from  underneath  it — you  will  not  be  alarmed  if  you 
hear  a  noise. 

Emily,  It  was  as  loud  as  the  report  of  a  gun,  and  the 
bladder  is  burst !  Pray  explain  how  the  air  is  concerned 
in  this  experiment. 

Mrs,  B,  It  is  the  effect  of  the  weight  of  the  atmo- 
sphere on  the  upper  surface  of  the  bladder,  when  I  had  ta- 
ken away  the  air  from  the  under  surface ;  so  that  there 
was  no  longer  any  re-action  to  counterbalance  the  pres- 
sure of  the  atmosphere  on  the  receiver.  You  observed 
how  the  bladder  was  pressed  inwards  by  the  weight  of 
the  external  air,  in  proportion  as  I  exhausted  the  receiver  : 
and  before  a  complete  vacuum  was  formed,  the  bladder, 

660.     Why  do  not  bodies  of  various  weights  in  the  atmosphere 

fall  in  the  same  time  ? 661.     What  does  the  fact  prove,  that 

Hght  bodies  are  retarded  by  the  air  in  falling  to  the  earth  .^ 662, 

How  may  it  be  shown  that  the  air  has  weight .'' 
14  * 


1(52     ON  THE  MECHANICAL  PROPERTIES  OF  AIR- 

unable  to  sustain  the  violence  of  the  pressure,  burst  with 
the  explosion  you  have  just  heard.* 

I  shall  now  show  you  an  experiment,  which  proves  the 
expansion  of  the  air,  contained  within  a  body  when  it  is 
relieved  from  the  pressure  of  the  external  air.  You 
would  not  imagine  that  there  was  any  air  contained  with- 
in this  shrivelled  apple,  by  its  appearance  ;  but  take  no- 
tice of  it  wlien  placed  within  a  receiver,  from  which  I 
shall  exhaust  the  air. 

Caroline,  How  •strange!  it  grows  quite  plump,  and 
looks  like  a  fresh-gathered  apple. 

Mrs,  B,  But  as  soon  as  I  let  the  air  again  into  the 
receiver,  the  apple  you  see  returns  to  its  shrivelled  state. 
When  I  took  away  the  pressure  of  the  atmosphere,  the  air 
within  the  apple  expanded  and  swelled  it  out  ;  but  the 
instant  the  atmospherical  air  was  restored,  the  expansion 
of  the  internal  air  was  checked  and  repressed,  and  the 
apple  shrunk  to  its  former  dimensions. 

You  may  make  a  similar  experiment  with  this  little 
bladder,  which  you  see  is  perfectly  flaccid  and  appears  to 
contain  no  air :  in  this  state  I  shall  tie  up  the  neck  of 
the  bladder,  so  that  whatever  air  remains  within  it  may 
not  escape,  and  then  place  it  under  the  receiver.  Now 
observe,  as  I  exhaust  the  receiver,  how  the  bladder  dis- 
tends ;  this  proceeds  from  the  great  dilatation  of  the 
small  quantity  of  air  which  was  enclosed  within  the  blad- 
der when  I  tied  it  up :  but  as  soon  as  I  let  the  air  into 
the  receiver,  that  which  the  bladder  contains,  condenses 


*  The  weight  of  the  atmosphere  can  also  be  ascertained  from 
the  following  experiments. — The  air  being  exhausted,  by  an  air- 
pump,  from  a  glass  receiver,  the  receiver  will  be  held  fast  by  the 
pressure  of  the  external  air.  If  a  small  receiver  be  placed  under 
a  larger  one,  and  the  air  be  exhaiirted  from  both,  the  larger  one 
v/ill  be  held  fast  by  the  pressure  of  external  air,  while  the  smaller 
one  will  be  easily  moved.  Or,  if  the  hand  be  placed  upon  a  small 
open  vessel  in  such  a  manner  as  to  close  its  upper  orifice,  it  will  be 
held  down  with  great  force. 

663.     What  experiments  named  in  the  note  prove  that  air  has 

iveight  f 664.    How  may  the  elasticity  or  expansive  power  of 

the  air  be  shown  ? 


lue? 


ON  THE  MECHANICAL  PROPERTIES  OP  AIR.  16S 

and  shrinks  into  its  small  compass  within  the  folds  of  the 
bladder.* 

Emily,  These  experiments  are  extremely  amusing, 
and  they  afford  clear  proofs  both  of  the  weight  and  elas- 
ticity of  the  air  ;  but  I  should  like  to  know  exactly  how 
much  the  air  weighs. 

Mrs.  B,  A  column  of  air  reaching  to  the  top  of  the 
atmosphere,  and  whose  base  is  a  square  inch,  weighs  151bs. 
when  the  air  is  heaviest  ;  therefore  every  square  inch  of 
our  bodies  sustains  a  weight  of  lolbs.  :  and  if  you  wish  to 
know  the  weight  of  the  whole  of  the  atmosphere,  you  must 
reckon  how  many  square  inches  there  are  on  the  surface 
of  the  globe,  and  multiply  them  by  15.t 

Emily,  But  are  there  no  means  of  ascertaining  the 
weight  of  a  small  quantity  of  air  ? 

Mrs,  B,  Nothing  more  easy.  I  shall  exhaust  the  air 
from  this  little  bottle  by  means  of  the  air  pump  :  and  hav- 
ing emptied  the  bottle  of  air,  or,  in  other  words,  pro- 
duced a  vacuum  within  it,  I  secure  it  by  turning  this  screw 
adapted  to  its  neck  :  we  may  now  find  the  exact  weight 
of  this  bottle,  by  putting  it  into  one  of  the  scales  of  a  ba- 
lance. It  weighs  you  see  just  two  ounces ;  but  when  I 
turn  the  screw,  so  as  to  admit  the  air  into  the  bottle,  the 
scale  which  contains  it  preponderates. 

Caroline,  No  doubt,  the  bottle  filled  with  air,  is  hea- 
vier than  the  bottle  void  of  air  ;  and  the  additional  weight 
required  to  bring  the  scales  again  to  a  balance,  must  be 
exactly  that  of  the  air  which  the  bottle  now  contains. 

*  If  a  tube,  closed  at  one  end,  be  inserted  at  its  open  end,  in  a 
vessel  of  water,  the  fluid  in  the  tube  will  not  rise  to  the  level  of  the 
water  in  the  vessel,  being  resisted  by  the  elastick  force  of  the  air 
within  the  tube.  It  is  on  this  principle  that  the  diving  bell  is 
formed.  ^^^ 


t  It  has  been  computed  that  the  pressure  of  the  atmosphere  on 
the  whole  surface  of  the  earth  is  equivalent  to  that  of  a  globe  of 
lead  sixty  miles  in  diameter. 

665.     How  much  does  a  column  of  air,  reaching  to  the  top  of  the 

atmosphere,  of  an  inch  in  diameter,  weigh  } 666.     How  great 

has  been  estimated  the  whole  'pressure  of  the  atmosphere  upon  the 

earth  f 667.     How  can  the  weight  of  a  small  quantity  of  air  be 

ascertained  ^ 


164  ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

Mrs,  B.  That  weight,  you  see,  is  almost  two  grains. 
The  dimensions  of  this  bottle  are  six  cubick  inches.  Six 
cubick  inches  of  air,  therefore,  at  the  temperature  of  this 
room,  weigh  nearly  two  grains. 

Caroline,  Why  do  you  observe  the  temperature  of 
the  room  in  estimating  the  weight  of  the  air  1 

Mrs,  B,  Because  heat  rarefies  air,  and  renders  it 
lighter  ;  therefore  the  warmer  the  air  is  which  you  weigh, 
the  lighter  it  will  be. 

If  you  should  now  be  desirous  of  knowing  the  spe- 
cifick  gravity  of  this  air,  we  need  only  fill  the  same  bottle 
with  water,  and  thus  obtain  the  weight  of  an  equal  quan- 
tity of  water — which  you  see  is  1515  grains;  now  by 
comparing  the  weight  of  water  to  that  of  air  we  find  it  to 
be  in  the  proportion  of  about  800  to  1. 

I  will  show  you  another  instance  of  the  weight  of  the 
atmosphere,  which  I  think  will  please  you  :  you  know 
what  a  barometer  is  ? 

Caroline,  It  is  an  instrument  which  indicates  the  state 
of  the  weather,  by  means  of  a  tube  of  quicksilver  ;  but 
how,  I  cannot  exactly  say. 

Mrs,  B.  It  is  by  showing  the  weight  of  the  atmo- 
sphere. The  barometer  is  an  instrument  extremely  simple 
in  its  construction  :  in  order  that  you  may  understand  it, 
I  will  show  you  how  it  is  made.  I  first  fill  a  glass  tube  A 
B,  (fig  .3,  plate  XIV.)  about  three  feet  in  length,  and  open 
only  at  one  end,  with  mercury ;  then  stopping  the  open 
end  with  my  finger,  I  immerse  it  in  a  cup  C,  containing 
a  little  mercury. 

Emily,  Part  of  the  mercury  which  was  in  the  tube, 
I  observe,  runs  down  into  the  cup  ;  but  why  does  not  the 
whole  of  it  subside  in  the  cup,  for  it  is  contrary  to  the  law 
of  the  equilibrium  of  fluids,  that  the  mercury  in  the  tube 
should  not  descend  to  a  level  with  that  in  the  cup. 

Mrs,  B.  The  mercury  that  has  fallen  from  the  tube 
into  the  cup,  has  left  a  vacant  space  in  the  upper  part  of 
the  tube,  to  which  the  air  cannot  gain  access  ;  this  space 
is   therefore  a  perfect    vacuum  ;  and    consequently  the 

668.     Why   is  it  necessary  in  this  experiment  to  observe  the 

temperature  of  the  room  in  which  it  is  made  ? 669.  How  much 

heavier  is  water  than  air  ? 670.     How  is  the  specifick  gravity 

of  air  determined? 671.  What  is  a  barometer  .^ 672.  Which 

figure  represents  a  barometer  ? 673.     How  is  the  weight  of  the 

atmosphere  determined  by  a  barometer  r 


ON  THE  MECHANICAL  PROPERTIES  OP  AIR.  165 

mercury  in  the  tube  is  relieved  from  the  pressure  of  the 
atmosphere,  whilst  that  in  the  cup  remains  exposed  to  it. 

Caroline,  Oh,  now  I  understand  it ;  the  pressure  of 
the  air  on  the  mercury  in  the  cup  forces  it  to  rise  in  the 
tube,  where  it  sustains  no  pressure. 

Emily,  Or  rather  supports  the  mercury  in  the  tube, 
and  prevents  it  from  falling. 

Mrs,  B,  That  comes  to  the  same  thing  ;  for  the  pow- 
er that  can  support  mercury  in  a  vacuum,  would  also  make 
it  ascend  when  it  met  with  a  v  acuum. 

Thus  you  see,  that  the  equilibrium  of  the  mercury  is 
destroyed  only  to  preserve  the  general  equilibrium  of 
fluids. 

Caroline,  But  this  simple  apparatus  is,  in  appearance, 
very  unlike  a  barometer. 

Mrs,  B,  It  is  all  that  is  essential  to  a  barometer. 
The  tube  and  the  cup  or  vase  are  fixed  on  a  board,  for 
the  convenience  of  suspending  it  ;  the  board  is  graduated 
for  the  purpose  of  ascertaining  the  height  at  which  the 
mercury  stands  in  the  tube  ;  and  the  small  moveable  me- 
tal plate  serves  to  show  that  height  with  greater  accuracy. 

Emily,  And  at  what  height  will  the  weight  of  the  at- 
mosphere sustain  the  mercury  ? 

Mrs,  B,  About  28  inches,  as  you  will  see  by  this 
barometer  ;  but  it  depends  upon  the  weight  of  the  atmo- 
sphere, which  varies  much  according  to  the  state  of  the 
weather.  The  greater  the  pressure  of  the  air  on  the  mer- 
cury in  the  cup,  the  higher  it  will  ascend  in  the  tube. 
Now  can  you  tell  me  whether  the  air  is  hesfVier  in  wet  or 
dry  weather  1 

Caroline.  Without  a  moment's  reflection,  the  air 
must  be  heaviest  in  wet  vveather.  It  is  so  depressing,  and 
makes  one  feel  so  heavy  ;  while  in  fine  weather,  I  feel  as 
light  as  a  feather,  and  as  brisk  as  a  bee. 

Mrs,  B,  Would  it  not  have  been  better  to  have  an- 
swered with  a  moment's  reflection,  Caroline  ?  It  would 
have  convinced  you,  that  the  air  must  be  heaviest  in  dry 
weather,  for  it  is  then,  that  the  mercury  is  found  to  rise 
in  the  tube,   and  consequently  the  mercury  in  the  cup 


674.     At  what  height  will  the  weight  of  the  atmospliere  sustain 

the  mercury  ? 675,     According  to  what  does  the  weight  of  the 

atmosphere  vary  ? 676.     When  is  the  air  the  heaviest,  in  v/et 

or  dry  weather  ? 


166  ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

must  be  most  pressed  by  the  air  :  and  you  know,  that  we 
estimate  the  dryness  and  fairness  of  the  weather,  by  the 
height  of  the  mercury  in  the  barometer, 

Caroline,  Why  then  does  the  air  feel  so  heavy  in  bad 
weather  ? 

Mrs.  IB,  Because  it  is  less  salubrious  when  impreg- 
nated with  damp.  The  langs  under  these  circumstances 
do  not  play  so  freely,  nor  does  the  blood  circulate  so  well  : 
thus  obstructions  are  frequently  occasioned  in  the  smaller 
vessels,  from  which   arise  colds,  asthmas,  agues,  fevers, 

&.C. 

Emily,  Since  the  atmosphere  diminishes  in  density  in 
the  upper  regions,  is  not  the  air  more  rare  upon  a  hill 
than  in  a  plain  ;  and  does  the  barometer  indicate  this 
difference  ? 

Mrs,  B,  Certainly.  The  hills  in  this  country  are  not 
sufficiently  elevated  to  produce  any  very  considerable  ef- 
fect on  the  barometer ;  but  this  instrument  is  so  exact  in 
its  indications,  that  it  is  used  for  the  purpose  of  measuring 
the  height  of  mountains,  and  of  estimating  the  elevation 
of  balloons. 

Emily.  And  is  no  inconvenience  experienced  from 
the  thinness  of  the  air  in  such  elevated  situations  ? 

Mrs.  B.  Oh,  yes  ;  frequently.  It  is  sometimes  op- 
pressive, from  being  insufficient  for  respiration ;  and  the 
expansion  which  takes  place  in  the  more  dense  air  con- 
tained within  the  body  is  often  painful  :  it  occasions  dis- 
tension, and  sometimes  causes  the  bursting  of  the  smaller 
blood-vessels  in  the  nose  and  ears.  Besides,  in  such  situ- 
ations, you  are  more  exposed  both  to  heat  and  cold  ;  for 
though  the  atmosphere  is  itself  transparent,  its  lower  re- 
gions abound  with  vapours  and  exhalations  from  the  earth, 
which  float  in  it,  and  act  in  some  degree  as  a  covering, 
vvhich  preserves  us  equally  from  the  intensity  of  the  sun's 
^ays,  and  from  the  severity  of  the  cold. 

Caroline.  Pray,  Mrs.  B.,  is  not  the  thermometer  con- 
structed on  the  same  principles  as  the  barometer  1 

Mrs.  B.  Not  at  all.  The  rise  and  fall  of  the  fluid 
in  the  thermometer  is  occasioned  by  the  expansive  power 

677.  Why  then  do  our  feelings  indicate  that  the  air  is  heaviest 
in  wet  weather,  if  that  is  not  the  fact  r G78.  Is  the  atmo- 
sphere of  the  same  density  on  a  hill  or  mountain  as  in  a  valley  ? 
-■ 679.  Does  a  person  in  elevated  situations  feel  any  inconveni- 
ence from  the  tliinness  of  the  atmosphere  ? 680.     What  causes 

the  rise  and  fall  of  the  fluid  in  the  thermometer  ? 


ON  THE  MECHANICAL  PROPERTIES  OF  AIR.  167 

of  heat,  and  the  condensation  produced  by  cold  ;  the  air  has 
no  access  to  it.  An  explanation  of  it  would,  therefore, 
be  irrelevant  to  our  present  subject. 

Emily.  I  have  been  reflecting,  that  since  it  is  the 
weight  of  the  atmosphere  which  supports  the  mercury  in 
the  tube  of  a  barometer,  it  would  support  a  column  of  any 
other  fluid  in  the  same  manner. 

Mrs,  B.  Certainly  ;  but  as  mercury  is  heavier  than 
all  other  fluids,  it  will  support  a  higher  column  of  any 
other  fluid  ;  for  two  fluids  are  in  equilibrium,  when  their 
height  varies  inversely  as  their  densities.  We  find  the 
weight  of  the  atmosphere  is  equal  to  sustaining  a  column 
of  water,  for  instance,  of  no  less  than  32  feet  above  its 
level. 

Caroline,  The  weight  of  the  atmosphere  is,  then,  as 
great  as  that  of  a  body  of  water  the  depth  of  32  feet  ? 

3Irs,  B,  Precisely  ;  for  a  column  of  air  of  the  height 
of  the  atmosphere,  is  equal  to  a  column  of  water  of  o2 
feet,  or  one  of  mercury  of  28  inches. 

The  common  pump  is  constructed  on  this  principle.  By 
the  act  of  pumping,  the  pressure  of  the  atmosphere  is  ta- 
ken off*  the  water,  which,  in  consequence,  rises. 

The  body  of  a  pump  consists  of  a  large  tube  or  pipe, 
whose  lower  end  is  immersed  in  the  water  which  it  is  de- 
signed to  raise.  A  kind  of  stopper,  called  a  piston,  is  fit- 
ted to  this  tube,  and  is  made  to  slide  up  and  down  it  by 
means  of  a  metallick  rod  fastened  to  the  centre  of  the  pis- 
ton. 

Emily,  Is  it  not  similar  to  the  syringe,  or  squirt,  with 
which  you  first  draw  in,  and  then  force  out  water  ? 

Mrs,  B,  It  is ;  but  you  know  that  we  do  not  wish  to 
force  the  water  out  of  the  pump  at  the  same  end  of  the 
pipe  at  which  we  draw  it  in.  The  intention  of  a  pump 
is  to  raise  water  from  a  spring  or  well ;  the  pipe  is  there- 
fore placed  perpendicularly  over  the  water  which  enters 
it  at  the  lower  extremity,  and  it  issues  at  a  horizontal 
spout  towards  the  upper  part  of  the  pump.     The  pump 

681.     Will  the  weight  of  the  atmosphere  support  other   fluids 

than  mercury  ? 082.     What  fluid  is  heaviest  ? 683.     When 

are  two  fluids  of  different  density  in  equilibrium  ? 684.     How 

high  a  column  of  water  will  the  weight  of  the  atmosphere  sustain  .'' 

685.     What  instrument  in  common  use  is  constructed  on  this 

principle  ? Q^Q.     What  causes  the  water  to  rise  in  a  pump  ? — — 

687.     How  is  a  common  pump  constructed  .'' 


168  ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

therefore,  is  rather  a  more  comphcated  piece  of  machine- 
ry than  the  syringe. 

Its  various  parts  are  delineated  in  this  figure :  (^^,  4. 
plate  XIV.)  A  B  is  the  pipe  or  body  of  the  pump,  P  the 
piston,  V  a  valve,  or  little  door  in  the  piston,  which  open- 
ing upwards,  admits  the  water  to  rise  through  it,  but  pre- 
vents its  returning,  and  Y  a  similar  valve  in  the  body  of 
the  pump. 

When  the  pump  is  in  a  state  of  inaction,  the  two  valves 
are  closed  by  their  own  weight ;  but  when,  by  drawing 
down  the  handle  of  the  pump,  the  piston  ascends,  it  raises 
a  column  of  air  which  rested  upon  it,  and  produces  a  va- 
cuum betw^een  the  piston  and  the  lower  valve  Y,  the  air 
beneath  this  valve,  which  is  immediately  over  the  surface 
of  the  water,  consequently  expands,  and  forces  its  way 
through  it ;  the  water,  then,  relieved  from  the  pressure 
of  the  air,  ascends  into  the  pump.  A  few  strokes  of  the 
handle  totally  excludes  the  air  from  the  body  of  the  pump, 
and  fills  it  with  water,  which,  having  passed  through  both 
the  valves,  runs  out  at  the  spout. 

Caroline,  I  understand  this  perfectly.  When  the 
piston  is  elevated,  the  air  and  the  water  successively  rise 
in  the  pump  ;  for  the  same  reason  as  the  mercury  rises  in 
the  barometer. 

Emily,  I  thought  that  water  was  drawn  up  into  a 
pump,  by  suction,  in  the  same  manner  as  water  may  be 
sucked  through  a  straw. 

Mrs,  B,  It  is  so,  into  the  body  of  the  pump  ;  for  the 
power  of  suction  is  no  other  than  that  of  producing  a  va- 
cuum over  one  part  of  the  liquid,  into  which  vacuum  the 
liquid  is  forced,  by  the  pressure  of  the  atmosphere  on 
another  part.  The  action  of  sucking  through  a  straw, 
consists  in  drawing  in  and  confining  the  breath,  so  as  to 
produce  a  vacuum  in  the  mouth  ;  in  consequence  of  which 
the  air  within  the  straw  rushes  into  the  mouth,  and  is  fol- 
lowed by  the  liquid,  into  which  the  lower  end  of  the 
straw  is  immersed.  The  principle,  you  see,  is  the  same, 
and  the  only  difference  consists  in  the  mode  of  producing 


688.     How  would  you  explain  the  pump,  by  reference  to  fig.  4, 

plate  XIV.  ? 689.     Is  the   power  of  suction,  and  that  which 

causes  water  to  rise  in  a  pump,  the  same  ? 690.     What  is  the 

power  of  suction  t 691.     In  what  consists  the  action  of  sucking 

liquid  through  a  straw  or  any  small  tube  ? 


ON    THE    MECHANICAL    PROPERTIES    OF    AIR.         169 

a  vacuum.  In  suction,  the  muscular  powers  answer  the 
purpose  of  the  piston  and  valves. 

Emily,  Water  cannot,  then,  be  raised  by  a  pump 
above  32  feet  ;  for  the  pressure  of  the  atmosphere  will 
not  sustain  a  column  of  water  above  that  height. 

Mrs.  B,  I  beg  your  pardon.  It  is  true  that  there 
must  never  be  so  great  a  distance  as  32  feet  from  the 
level  of  the  water  in  the  well,  to  the  valve  in  the  piston, 
otherwise  the  water  would  not  rise  through  that  valve  ; 
but  when  once  the  water  has  passed  that  opening,  it  is  no 
longer  the  pressure  of  air  on  the  rcvservoir  which  makes  it 
ascend  ;  it  is  raised  by  lifting  it  up,  as  you  would  raise 
it  in  a  bucket,  of  which  the  piston  formed  the  bottom. 
This  common  pump  is,  therefore,  called  the  sucking,  or 
lifting-pump,  as  it  is  constructed  on  both  these  principles. 
There  is  another  sort  of  pump,  called  the  forcing-pump  : 
it  consists  of  a  forcing  power  added  to  the  sucking  part  of 
the  pump.  This  additional  power  is  exactly  on  the  prin- 
ciple of  the  syringe  :  by  raising  the  piston  you  draw  the 
water  into  the  pump,  and  by  descending  it  you  force  the 
water  out. 

Caroline.  But  the  water  must  be  forced  out  at  the 
upper  part  of  the  pump  ;  and  I  cannot  conceive  how  that 
can  be  done  by  descending  the  piston. 

Mrs,  B,  Figure  5,  plate  XIV.  will  explain  the  diffi- 
culty. The  large  pipe  A  B  represents  the  sucking  part 
of  the  pump,  which  differs  from  the  lifting-pump,  only  in 
its  piston  P  being  unfurnished  with  a  valve,  in  consequence 
of  which  the  water  cannot  rise  above  it.  When,  there- 
fore, the  piston  descends,  it  shuts  the  valve  Y,  and  forces 
the  water  (which  has  no  other  vent)  into  the  pipe  D  :  this 
is  likewise  furnished  with  a  valve  V,  which,  opening  out- 
wards, admits  the  water,  but  prevents  its  return. 

The  water  is  thus  first  raised  in  the  pump,  and  then 
forced  into  the  pipe,  by  the  alternate  ascending  and  de- 
scending motion  of  the  piston,  after  a  few  strokes  of  the 


692.     What  in  auction  answer  the  purpose  of  the  piston  and 

valves  of  the  pump  ? 693      Can  water  be  raised   in  a  pump 

more  tiian  32  feet  ? 6i)4.     How  can  it,  if  the  weight  of  the  at- 
mosphere  is  only  equal  to  a  column  of  water  of  that  height? 

695      Of  what  does  the  forcing  pump  consist  <' 696.     Which 

,  figure  represents  the  forcing  pump  ? 697.     How  would  you  ex- 
plain the  forcing  pump  by  the  figure  ? 
15 


170  ON  WIND  AND  SOUND. 

handle  to  fill  the  pipe,  from  whence  the  water  issues  at 
the  spout. 

It  is  now  time  to  conclude  our  lesson.  When  next  we 
meet,  I  shall  give  you  some  account  of  wind,  and  of 
sound,  which  will  terminate  our  observations  on  elastick 
fluids. 

Caroline.  And  I  shall  run  into  the  garden,  to  have 
the  pleasure  of  pumping,  now  that  I  understand  the  con- 
struction of  a  pump. 

Mrs.  B.  And,  to-morrow  I  hope  you  will  be  able  to 
tell  me,  whether  it  is  a  forcing  or  a  common  lifting  pump. 


CONVERSATION  XIII. 

ON  WIND  AND  SOUND. 

Of  Wind  in  General;  Of  the  Trade  Wind;  Of  the 
Periodical  Trade  Winds ;  Of  the  Aerial  Tides ;  Of 
Sounds  in  General ;  Of  Sonorous  Bodies ;  Of  Musicat 
Sounds;   Of  Concord  or  Harmony,  and  Melody. 

MRS.  B. 

Well,  Caroline,  have  you  ascertained  what  kind  of 
pump  you  have   in  your  garden  ? 

Caroline.  I  think  it  must  be  merely  a  lifting-pump, 
because  no  more  force  is  required  to  raise  the  handle  than 
is  necessary  to  lift  its  weight ;  and  in  a  forcing  pump,  by 
raising  the  handle,  you  force  the  water  into  the  smaller 
pipe,  and  the  resistance  the  water  offers  must  require  an 
exertion  of  strength  to  overcome  it. 

Mrs.  B.  I  make  no  doubt  you  are  right;  for  lifting 
pumps,  being  simple  in  their  construction,  are  by  far  the 
most  common. 

I  have  promised  to-day  to  give  you  some  account  of 
the  nature  of  wind.  Wind  is  nothing  more  than  the  mo- 
tion of  a  stream  or  current  of  air,  generally  produced  by 
a  partial  change  of  temperature  in  the  atmosphere  ;  for 
when  any  one  part  is  more  heated  than  the  rest,  that  part 
is  rarefied  ;  the  equilibrium  is  destroyed,  and  the  air  in 
consequence  rises.     When   this   happens,   there   neces- 

698.     What  is  wind  ? 699,     How  is  the  air  put  in  motion  so 

as  to  produce  wind  ^ 


ON  WIND  AND  SOUND.  171 

sariiy  follows  a  motion  of  the  surrounding  air  towards  that 
part,  in  order  to  restore  it  ;  this  spot,  therefore,  receives 
winds  from  every  quarter.  Those  who  live  to  the  north 
of  it  experience  a  north  wind  ;  those  to  the  south,  a  south 
wind  : — do  you  comprehend  this  ?* 

Caroline,  Perfectly.  But  what  sort  of  weather  must 
those  people  have  who  live  on  the  spot  where  these  winds 
meet  and  interfere  ? 

Mrs,  B.  They  have  turbulent  and  boisterous  wea- 
ther, whirlwinds,  hurricanes,  rain,  lightning,  thunder,  &c. 
This  stormy  v/eather  occurs  most  frequently  in  the  torrid 
zone,  where  the  heat  is  greatest :  the  air,  being  more  ra- 
refied there  than  in  any  other  part  of  the  globe,  is  light- 
er, and  consequently  ascends ;  whilst  the  air  about  the 
polar  regions  is  continually  flowing  from  the  poles  to  re- 
store the  equilibrium.  ^ 

Caroline,  This  motion  of  the  air  would  produce  a  re- 
gular and  constant  north  wind  to  the  inhabitants  of  the 
northern  hemisphere  ;  and  a  south  wind  to  those  of  the 
southern  hemisphere  ;  and  continual  storms  at  the  equa- 
tor, where  these  two  adverse  winds  would  meet. 

Mrs.  B,  These  winds  do  not  meet,  for  they  each 
change  their  direction  before  they  reach  the  equator. 
The  sun,  in  moving  over  the  equatorial  regions  from  east 
to  west,  rarefies  the  air  as  it  passes,  and  causes  the  den- 
ser eastern  air  to  flow  westwards,  in  order  to  restore  the 
equilibrium  ;  thus  producing  a  regular  east  wind  about 
the  equator. 

Caroline,  The  air  from  the  west,  then,  constantly 
goes  to  meet  the  sun,  and  repair  the  disturbance  which 

*  Fill  a  large  dish  with  cold  water  ;  into  the  middle  of  this  put  a 
waiter,  filled  with  warm  water.  The  first  will  represent  the  ocean 
and  the  other  an  island,  rarefying-  the  air  above  it.  Blow  out  a  wax 
candle,  and  if  the  air  be  still,  on  applying  it  successively  to  every 
side  of  the  dish,  the  smoke  will  be  seen  to  move  towards  the  plate. 
— -Again,  if  the  ambient  water  be  warmed  and  the  plate  be  filled 
with  cold  water,  let  the  wick  of  smoking  candles  be  held  over  the 
plate,  and  the  contrary  will  happen. 

700.  What  illustration  of  wind  produced  by  change  of  tempera- 
ture is  given  in  the   note? 701.     What   is  the  consequence 

when  winds  from   different  quarters  meet  or  interfere  ? 702. 

Where  does  this  mostly  happen  ? 703.     Why  does  this  mostly 

happen  in  the  torrid  zone  ? 704.     What  regular  wind  prevaifs 

about  the  equator  ? 705.     Why  is  there  a  regular  east  wind  at 

and  near  the  equator  ^ 


172  ON  WIND  AND  SOUND. 

his  beams  have  produced  in  the  equilibrium  of  the  atmo- 
sphere. But  I  wonder  how  you  will  reconcile  these  va- 
rious winds,  Mrs.  B.  ;  you  first  led  me  to  suppose  there 
was  a  constant  struggle  between  opposite  winds  at  the 
equator  producing  storm  and  tempest ;  but  now  I  hear  of 
one  regular  invariable  wind,  which  must  naturally  be  at- 
tended by  calm  weather. 

Emily,  I  think  I  comprehend  it  :  do  not  these  winds 
from  the  north  and  south  combine  with  the  easterly  wind 
about  the  equator,  and  form  what  are  called  the  trade- 
winds  ? 

Mrs,  B,  Just  so,  my  dear.  The  composition  of  the 
two  winds  north  and  east,  produces  a  constant  north-cast 
wind  ;  and  that  of  the  two  winds  south  and  east,  produces 
a  regular  south-east  wind  :  these  winds  extend  to  about 
thirty  degrees  on  each  side  of  the  equator,  the  regions  fur- 
ther distant  from  it  experiencing  only  their  respective 
north  and  south  winds.* 

Caroline,  But,  Mrs.  B.,  if  the  air  is  constantly  flow- 
ing from  the  poles  to  the  torrid  zone,  there  must  be  a  de- 
ficiency of  air  in  the  polar  regions  ? 

Mrs,  B,  The  light  air  about  the  equator,  which  ex- 
pands and  rises  into  the  upper  regions  of  the  atmosphere, 
ultimately  flows  from  thence  back  to  the  poles,  to  restore 
the  equilibrium :  if  it  were  not  for  this  resource,  the  po- 
lar atmospherick  regions  would  soon  be  exhausted  by  the 
stream  of  air,  which,  in  the  lower  strata  of  the  atmosphere, 
they  are  constantly  sending  towards  the  equator. 

Caroline,  There  is  then  a  sort  of  circulation  of  air  in 
the  atmosphere  ;  the  air  in  the  lower  strata  flowing  from 
the  poles  towards  the  equator,  and  in  the  upper  strata  flow- 
ing back  from  the  equator  towards  the  poles. 


*^  On  the  coast  of  America,  the  trade  winds  are  felt  as  far  as  forty 
degrees  from  the  equator.  By  the  aid  of  these  winds,  vessels  sailing 
from  Mexico  to  the  Philippine  islands,  often  finish  a  voyage,  nearly 
equal  to  half  the  circumference  of  the  globe,  in  60  days,  without 
altering  their  course,  or  chnnginff  a  sail.  But  in  returning,  they 
are  obliged  to  go  north,  beyond  the  limits  of  the  trade  winds. 

704.     How  are  the  trade  winds  occasioned  ?^ 705.     How  far 

on  each  side  of  the  equator  do  these  winds  extend  r 706.     IVhat 

is  said  of  the  trade  winds  on  the  coast  of  America  ? 707.    What 

fact  is  mentioned  of  vessels  sailing  from  Mexico  to  the  Philippine 

islands  9 .-708.  Why  do  not  the  polar  regions  become  exhausted 

of  air,  if  it  is  continually  blowing  from  them  to  the  equator  ? 


ON  WIND  AND  SOUND.  ITS 

Mrs,  B.  Exactly.  I  can  show  you  an  example  of  this 
circulation  on  a  small  scale.  The  air  of  this  room  being 
more  rarefied  than  the  external  air,  a  wind  or  current  of 
air  is  pouring  in  from  the  crevices  of  the  windows  and 
doors,  to  restore  the  equilibrium  ;  but  the  light  air  with 
which  the  room  is  filled  must  find  some  vent,  in  order  to 
make  way  for  the  heavy  air  which  enters.  If  you  set  the 
door  a-jar,  and  hold  a  candle  near  the  upper  part  of  it,  you 
will  find  that  the  flame  will  be  blown  outwards,  showing 
that  there  is  a  current  of  air  flowing  out  from  the  upper 
part  of  the  room.  Now  place  the  candle  on  the  floor 
close  by  the  door,  and  you  will  perceive,  by  the  inclina- 
tion of  the  flame,  that  there  is  also  a  current  of  air  setting 
into  the  room. 

Caroline.  It  is  just  so ;  the  upper  current  is  the  warm 
light  air,  which  is  driven  out  to  make  way  for  the  stream 
of  cold  dense  air  which  enters  the  room  lower  down. 

Emily.  I  have  heard,  Mrs.  B.,  that  the  periodical 
winds  are  not  so  regular  on  land  as  at  sea ;  what  is  the 
reason  of  that  ? 

Mrs,  B,  The  land  reflects  into  the  atmosphere  a 
much  greater  quantity  of  the  sun's  rays  than  the  water  ; 
therefore  that  part  of  the  atmosphere  which  is  over  the 
land  is  more  heated  and  rarefied  than  that  which  is  over 
the  sea :  this  occasions  the  wind  to  set  in  upon  the  land, 
as  we  find  that  it  regularly  does  on  the  coast  of  Guinea, 
and  other  countries  in  the  torrid  zone. 

Emily,  I  have  heard  much  of  the  violent  tempests  oc- 
casioned by  the  breaking  up  of  the  monsoons  ;  are  not 
they  also  regular  trade  winds  ? 

Mrs,  B,  They  are  called  periodical  trade-winds,  as 
they  change  their  course  every  half-year.  This  varia- 
tion is  produced  by  the  earth's  annual  course  round  the 
sun,  when  the  north  pole  is  inclined  towards  that  lumina- 
ry one  half  of  the  year,  the  south  pole  the  other  half.  Du- 
ring the  summer  of  the  northern  hemisphere,  the  countries 
of  Arabia,  Persia,  India,  and  China,  are  much  heated, 


709.     What  familiar  illustration  can  you  give  of  the  circulation 
of  the  air,  first  from  the  poles  to  the  equator,  and  then  rising  and 

returning  to  the  poles  ? 710.     Why  are  the  periodical  winds 

more  regular  at  sea  than  on  land  ? 711.     What  winds  are  call- 
ed monsoons  ^ 712.     How  is  the  variation  of  the  monsoone 

produced  ? 

15* 


174  ON  WIND  AND  SOUND. 

and  reflect  great  quantities  of  the  sun's  rays  into  the  at- 
mosphere, by  which  it  becomes  extremely  rarefied,  and 
the  equilibrium  consequently  destroyed.  In  order  to  re- 
store it,  the  air  from  the  equatorial  southern  regions,  where 
it  is  colder,  (as  well  as  from  the  colder  northern  parts,) 
must  necessarily  have  a  motion  towards  those  parts.  The 
current *of  air  from  the  equatorial  regions  produces  the 
trade-winds  for  the  first  six  months,  in  all  the  seas  between 
the  heated  continent  of  Asia,  and  the  equator.  The  other 
six  months,  when  it  is  summer  in  the  southern  hemi- 
sphere, the  ocean  and  countries  towards  the  southern 
tropick  are  most  heated,  and  the  air  over  those  parts  most 
rarefied :  then  the  air  about  the  equator  alters  its  course, 
and  flows  exactly  in  an  opposite  direction.* 

Caroline,  This  explanation  of  the  monsoons  is  very 
curious ;  but  what  does  their  breaking  up  mean  ] 

Mr$,  B,  It  is  the  name  given  by  sailors  to  the  shifting 
of  the  periodical  winds  ;  they  do  not  change  their  course 
suddenly,  but  by  degrees,  as  the  sun  moves  from  one  he- 
misphere to  the  other :  this  change  is  usually  attended  by 
storms  and  hurricanes,  very  dangerous  for  shipping  ;  so 
that  those  seas  are  seldom  navigated  at  the  season  of  the 
equinox. 

Emily.  I  think  I  understand  the  winds  in  the  torrid 
zone  perfectly  well ;  but  what  is  it  that  occasions  the 
great  variety  of  winds  which  occur  in  the  temperate  zones  1 
for  according  to  your  theory,  there  should  be  only  north 
and  south  winds  in  those  climates. 

Mrs.  B.  Since  so  large  a  portion  of  the  atmosphere  as 
is  over  the  torrid  zone  is  in  continued  agitation,  these  agi- 
tations in  an  elastick  fluid,  which  yields  to  the  slightest 
impression,  must  extend  every  way  to  a  great  distance  ; 
the  air,  therefore,  in  all  climates,  will  suffer  more  or  less 
perturbation,  according  to  the  situation  of  the  country, 
the  position  of  mountains,  valleys,  and  a  variety  of  other 
causes  :  hence  it  is  easy  to  conceive,  that  almost  every 
climate  must  be  liable  to  variable  winds. 

*  The  south-west  monsoon,  which  blows  from  April  to  October, 
brings  with  it  floods  of  rain,  and  dreadful  tempests.  During  the 
rest  of  the  year,  the  north-east  monsoon  produces  a  dry  and  agree- 
able state  of  the  air. 

713.      What  effect  do  the  monsoons  have  on  the  weather  ? 

714.  What  does  the  breaking  up  of  the  monsoons  mean  ? 

715.  What  is  it  that  occasions  the  great  variety  of  winds  which 
occur  in  the  temperate  zones  ' 


•^     ON  WIND  AND  SOUND.  175 

on  the  sea-shore,  there  is  almost  always  a  gentle  sea-breeze 
setting  in  on  the  land  on  a  summer's  evening,  to  restore  the 
equilibrium  which  had  been  disturbed  by  reflections  from 
the  heated  surface  of  the  shore  during  the  day  ;  and  when 
night  has  cooled  the  land,  and  condensed  the  air,  we  ge- 
nerally find  it,  towards  morning,  flowing  back  towards  the 
sea. 

Caroline.  I  have  observed  that  the  wind,  whichever 
way  it  blows,  almost  always  falls  about  sun-set. 

Mrs,  B.  Because  the  rarefaction  of  air  in  the  particu- 
lar spot  which  produces  the  wind,  diminishes  as  the  sun 
declines,  and  consequently  the  velocity  of  the  wind  abates. 

Emily,  Since  the  air  is  a  gravitating  fluid,  is  it  not 
affected  by  the  attraction  of  the  moon  and  the  sun,  in  the 
same  manner  as  the  waters  ? 

Mrs,  B,  Undoubtedly ;  but  the  aerial  tides  are  as 
much  greater  than  those  of  water,  as  the  density  of  water 
exceeds  that  of  air,  which,  as  you  may  recollect,  we  found 
to  be  about  800  to  1. 

Caroline,  What  a  prodigious  protuberance  that  must 
occasion  ;  how  much  the  weight  of  such  a  column  of  air 
must  raise  the  mercury  in  the  barometer  ! 

Emily,  As  this  enormous  tide  of  air  is  drawn  up  and 
supported,  as  it  were  by  the  moon,  its  weight  and  pres- 
sure, I  should  suppose,  would  be  rather  diminished  than 
increased  ? 

Mrs,  B,  The  weight  of  the  atmosphere  is  neither  in- 
creased nor  diminished  by  the  aerial  tides.  The  moon's 
attraction  augments  the  bulk  as  much  as  it  diminishes 
the  weight  of  the  column  of  air  ;  these  effects,  therefore, 
counterbalancing  each  other,  the  aerial  tides  do  not  affect 
the  barometer. 

Caroline.     I  do  not  quite  understand  that. 

Mrs.  B.  Let  us  suppose  that  the  additional  bulk  of 
air  at  high  tide  raises  the  barometer  one  inch  ;  and  on 
the  other  hand,  that  the  support  which  the  moon's  attract- 
tion  affords  the  air,  diminishes  its  weight  or  pressure,  so 
as  to  occasion  the  mercury  to  fall  one  inch  ;  under  these 


716.     What  are   the  sea-breezes  as  they  are  termed  ? 717. 

Why  does  the  wind  generally  subside  at  the  going  down  of  the  sun  ? 

71 8.     Does  the  moon  have  any  effect  on  the  wind  ? 719. 

How  much  greater  are  the  aerial  tides  than  those  of  water  ? — — 
720.     Why  do  not  the  aerial  tides  aifect  the  baromete  ? 


176  ON  WIN'b  AND  SOUND. 

circumstances  the  mercury  must  remain  stationary.  Thus 
you  see,  that  we  can  never  be  sensible  of  aerial  tides  by 
the  barometer,  on  account  of  the  equality  of  pressure  of 
the  atmosphere,  whatever  be  its  height. 

The  existence  of  aerial  tides  is  not,  however,  hypo- 
thetical ;  it  is  proved  by  the  effect  they  produce  on  the 
apparent  position  of  the  heavenly  bodies  ;  but  this  I  can- 
not explain  to  you,  till  you  understand  the  properties  of 
light.* 

Emily.     And  when  shall  we  learn  them  1 

Mrs,  B,  I  shall  first  explain  to  you  the  nature  of 
sound,  which  is  intimately  connected  with  that  of  air  ; 
and  I  think  at  our  next  meeting  we  may  enter  upon  the 
subject  of  opticks. 

We  have  now  considered  the  effects  produced  by  the 
wide  and  extended  agitation  of  the  air  ;  but  there  is  ano- 
ther kind  of  agitation  of  which  the  air  is  susceptible — a 
sort  of  vibratory,  trembling  motion,  which,  striking  on  the 
drum  of  the  ear,  produces  sound.i 

Caroline.  Is  not  sound  produced  by  solid  bodies? 
The  voice  of  animals,  the  ringing  of  bells,  musical  in- 
struments, are  all  solid  bodies.  I  know  of  no  sound  but 
that  of  the  wind  which  is  produced  by  the  air. 

Mrs,  B,  Sound,  I  assure  you,  results  from  a  tremu- 
lous motion  of  the  air  ;  and  the  sonorous  bodies  you  enu- 
merate, are  merely  the  instruments  by  which  that  peculiar 
species  of  motion  is  communicated  to  the  air. 


*  The  quality  of  winds  is  affected  by  the  countries  over  which 
they  pass  ;  and  they  are  sometimes  rendered  pestilential  by  the 
heat  of  deserts,  or  the  putrid  exhalations  of  marshes  and  lakes. 
Thus,  from  the  deserts  of  Africa,  Arabia,  and  the  neighbouring 
countries,  a  hot  wind  blows,  called  Samiel  or  Simoom^  which  some- 
times produces  instant  death.  A  similar  wind  blows  from  the  Sa- 
hara, upon  the  western  coast  of  Africa,  called  the  Harmattan^  pro- 
ducing a  dryness  and  heat  which  is  almost  insupportable,  and 
scorching  like  the  blasts  of  a  furnace. 

t  The  science  which  treats  of  the  nature,  phenomena,  and  laws 
of  sound,  is  called  Acousticks.  This  science  is  particularly  inte- 
resting and  valuable  from  its  extending  to  the  theory  of  musical  con- 
cord and  harmony. 


721.     By  what  is  the  quality  of  winds  affected  ? 722.     What 

facts  are  stated  in  the  notes  illustrating  the  effects  thus  produced 
on  the  wind  ? 723.     How  is  sound  produced  ^ 


ON  WIND  AND  SOUND.  177 

« 

Caroline,  What !  when  I  ring  this  little  bell,  is  it  the 
air  that  sounds,  and  not  the  bell  ? 

Mrs.  B.  Both  the  bell  and  the  air  are  concerned  in 
the  production  of  sound.  But  sound,  strictly  speaking, 
is  a  perception  excited  in  the  mind  by  the  motion  of  the 
air  on  the  nerves  of  the  ear  ;  the  air,  therefore,  as  well  as 
the  sonorous  bodies  which  put  it  in  motion,  is  only  the 
cause  of  sound,  the  immediate  effect  is  produced  by  the 
sense  of  hearing  :  for,  without  this  sense,  there  would  be 
no  sound. 

Emihj,  I  can  with  difficulty  conceive  that.  A  person 
born  deaf,  it  is  true,  has  no  idea  of  sound,  because  he  hears 
none  ;  yet  that  does  not  prevent  the  real  existence  of 
sound,  as  all  those  who  are  not  deaf  can  testify. 

Mrs,  B.  I  do  not  doubt  the  existence  of  sound  to  all 
those  who  possess  the  sense  of  hearing  ;  but  it  exists 
neither  in  the  sonorous  body  nor  in  the  air,  but  in  the 
mind  of  the  person  whose  ear  is  struck  by  the  vibratory 
motion  of  the  air,  produced  by  a  sonorous  body. 

To  convince  you  that  sound  does  not  exist  in  sonorous 
bodies,  but  that  air  or  some  other  vehicle  is  necessary  to 
its  production,  endeavour  to  ring  the  little  bell,  after  I 
have  suspended  it  under  a  receiver  in  the  air-pump,  from 
which  I  shall  exhaust  the  air 

Caroline.  This  is  indeed  very  strange  :  though  I  agi- 
tate it  so  violently,  it  does  not  produce  the  least  sound. 

Mrs.  B.  By  exhausting  the  receiver,  I  have  cut  off 
the  communication  between  the  air  and  the  bell ;  the  lat- 
ter, therefore,  cannot  impart  its  motion  to  the  air. 

Caroline.  Are  you  sure  that  it  is  not  the  glass,  which 
covers  the  bell,  that  prevents  our  hearing  it  1 

Mrs.  B.  That  you  may  easily  ascertain  by  letting  the 
air  into  the  receiver,  and  then  ringing  the  bell. 

Caroline.  Very  true  :  I  can  hear  it  now  almost  as  loud 
as  if  the  glass  did  not  cover  it ;  and  I  can  no  longer  doubt 
but  that  air  is  necessary  to  the  production  of  sound. 

Mrs.  B.  Not  absolutely  necessary,  though  by  far  the 
most  common  vehicle  of  sound.  Liquids,  as  well  as  air, 
are  capable  of  conveying  the  vibratory  motion  of  a  sono- 


724.     What  is  sound,  strictly  speaking  ? 725.     How  can  it 

be  shown  that  air  is  necessary  in  the  production  of  sound  ? 726. 

Why  cannot  a  bell  be  heard  in  an  exhausted  receiver  ? 727. 

Is  the  atmosphere  the  only  conductor  of  sound  ? 


178  ON  WIND  AND  SOUND. 

reus  body  to  the  organ  of  hearing ;  as  sound  can  be  heard 
under  water.  Solid  bodies  also  convey  sound,  as  I  can 
soon  convince  you  by  a  very  simple  experiment.  I  shall 
fasten  this  string  by  tlie  middle  round  the  poker ;  now 
raise  the  poker  from  the  ground  by  the  two  ends  of  the 
string,  and  hold  one  to  each  of  your  ears  : — I  shall  now 
strike  the  poker  with  a  key,  and  you  will  find  that  the 
sound  is  conveyed  to  the  ear  by  means  of  the  strings,  in  a 
much  more  perfect  manner  than  if  it  had  no  other  v^ehicle 
than  the  air. 

Caroline,  That  it  is,  certainly,  for  I  am  almost  stun- 
ned by  the  noise.  But  what  is  a  sonorous  body,  Mrs.  B.  ] 
for  all  bodies  are  capable  of  producing  some  kind  of  sound 
by  the  motion  they  communicate  to  the  air. 

Mrs,  B.  Those  bodies  are  called  sonorous,  which  pro- 
duce clear,  distinct,  regular,  and  durable  sounds,  such  as 
a  bell,  a  drum,  musical  strings,  wind  instruments,  6lc, 
They  owe  this  property  to  their  elasticity  ;  for  an  elastick 
body,  after  having  been  struck,  not  only  returns  to  its 
former  situation,  but  having  acquired  momentum  by  its  ve- 
locity, like  the  pendulum,  it  springs  out  on  the  opposite 
side.  If  I  draw  the  siring  A  B,  which  is  made  fast  at 
both  ends,  to  C,  it  will  not  only  return  to  its  original  po- 
sition, but  proceed  onwards  to  D. 

This  is  its  first  vibration,  at  the  end  of  which  it  will  re- 
tain sufficient  velocity  to  bring  it  to  E,  and  back  again  to 
F,  which  constitutes  its  second  vibration  ;  the  third  vibra- 
tion will  carry  it  only  to  G  and  H,  and  so  on  till  the  re- 
sistance of  the  air  destroys  its  motion. 

The  vibration  of  a  sonorous  body  gives  a  tremulous  mo- 
tion to  the  air  around  it,  very  similar  to  the  motion  com- 
municated to  smooth  water  when  a  stone  is  thrown  into  it. 
This  first  produces  a  small  circular  wave  around  the 
spot  in  which  the  stone  falls  ;  the  'wave  spreads,  and 
gradually  communicates  its  motion  to  the  adjacent  wa- 
ters, producing  similar  waves  to  a  considerable  extent. 
The  same  kind  of  waves  is  produced  in  the  air  by  the 


7:28.     What  besides  air  convey  the  vibratory  motion  of  sonorous 

bodies  ?- 729.     What  bodies  are  called  sonorous  ? 730.     To 

what  do  they  owe  their  sonorous  property  ? 731.     How  would 

you  explain  Fig.  6,  plate  XIV.  as  illustrating  the  production  of 

sound  .•' 732.     To  what  is  the  tremulous  motion,  given  to  the 

air  by  a  sonorous  body,  compared  ? 


i 


ON  WIND  AND  SOUND.  179 

motion  of  a  sonorous  body,  but  with  this  difference,  that 
as  air  is  an  elastick  fluid,  the  motion  does  not  consist  of 
regularly  extending  waves,  but  of  vibrations,  and  are  com- 
posed of  amotion  forwards  and  backwards,  similar  to  those 
of  the  sonorous  body.  They  differ  also  in  the  one  taking 
place  in  a  plane,  the  other  in  all  directions.  The  aerial 
undulations  being  spherical. 

Emily.  But  if  the  air  moves  backwards  as  well  as  for- 
wards, how  can  its  motion  extend  so  as  to  convey  sound 
to  a  distance. 

3Irs.  B.  The  first  sphere  of  undulations  which  are 
produced  immediately  around  the  sonorous  body,  by 
pressing  against  the  contiguous  air,  condenses  it.  The 
condensed  air,  though  impelled  forward  by  the  pressure, 
re-acts  on  the  first  set  of  undulations,  driving  them  back 
again.  The  second  set  of  undulations  which  have  been 
put  in  motion,  in  their  turn  communicate  their  motion, 
and  are  themselves  driven  back  by  re-action.  Thus  there 
is  a  succession  of  waves  in  the  air,  corresponding  with  the 
succession  of  waves  in  the  water. 

Caroline,  The  vibrations  of  sound  must  extend  much 
further  than  the  circular  waves  in  water,  since  sound  is 
conveyed  to  a  great  distance. 

3Irs,  B,  The  air  is  a  fluid  so  much  less  dense  than 
water,  that  motion  is  more  easily  communicated  to  it. 
The  report  of  a  cannon  produces  vibrations  of  the  air 
which  extend  to  several  miles  around. 

Emihj,  Distant  sound  takes  some  time  to  reach  us, 
since  it  is  produced  at  the  moment  the  cannon  is  fired  ; 
and  we  see  the  light  of  the  flash  long  before  we  hear  the 
report. 

Mrs,  B.  The  air  is  immediately  put  in  motion  by  the 
firing  of  a  cannon;  but  it  requires  time  for  the  vibrations 
to  extend  to  any  distant  spot.  The  velocity  of  sound  is 
computed  to  be  at  the  rate  of  1142  feet  in  a  second. 

Caroline.  With  what  astonishing  rapidity  the  vibra- 
tions must  be  communicated  !  But  the  velocity  of  sound 
varies,  I  suppose,  with  that  of  the  air  which  conveys  it. 
If  the  wind  sets  towards  us  from  the  cannon,  we  must 
hear  the  report  sooner  than  if  it  set  the  other  way. 

7:33     If  the  air  reverberate,  how  can  its  motion  extend  so  as  to 

convey  sound  to  a  distance  ? 734.     Why  is  motion  more  easily 

communicated  to  air  than  to  water  ? 735.     Why  do  we  see  the 

flash  of  a  cannon,  at    a  distance,  before  we  hear  the  report  ? » 

736.     What  is  the  computed  velocity  of  sound  .'' 


180  ON  WIND  AND  SOUND. 

Mrs.  B,  The  direction  of  the  wind  makes  less  diffe- 
rence in  the  velocity  of  sound  than  you  would  imagine. 
If  the  wind  sets  from  us,  it  bears  most  of  the  aerial  waves 
away,  and  renders  the  sound  fainter  ;  but  it  is  not  very 
considerably  longer  in  reaching  the  ear  than  if  the  wind 
blew  towards  us.  This  uniform  velocity  of  sound  enables 
us  to  determine  the  distance  of  the  object  from  which  it 
proceeds ;  as  that  of  a  vessel  at  sea  firing  a  cannon,  or 
that  of  a  thunder  cloud.  If  we  do  not  hear  tlie  thunder 
till  half  a  minute  after  we  see  the  lightning,  we  conclude 
the  cloud  to  be  at  the  distance  of  six  miles  and  a  half. 

Emily.     Pray  how  is  the  sound  of  an  echo  produced  ? 

Mrs.  B.  When  the  aerial  vibrations  meet  with  an  ob- 
stacle, having  a  hard  and  regular  surface,  such  as  a  wall, 
or  rock,  they  are  reflected  back  to  the  ear  and  produce 
the  same  sound  a  second  time  ;  but  the  sound  will  then 
appear  to  proceed  from  the  object  by  which  it  is  reflected. 
If  the  vibrations  fall  perpendicularly  on  the  obstacle,  they 
are  reflected  back  in  the  same  line  ;  if  obliquely,  the  sound 
returns  obliquely  in  the  opposite  direction,  the  angle  of 
reflection  being  equal  to  the  angle  of  incidence. 

Caroline.  Oh,  then,  Emily,  I  now  understand  why  the 
echo  of  my  voice  behind  our  house  is  heard  so  much 
plainer  by  you  than  it  is  by  me,  when  we  stand  at  oppo- 
site ends  of  the  gravel  walk.  My  voice,  or  rather,  I  should 
say,  the  vibrations  of  air  it  occasions,  fall  obliquely  on  the 
wall  of  the  house,  and  are  reflected  by  it  to  the  opposite 
end  of  the  gravel  walk. 

Emily.  Very  true ;  and  we  have  observed  that  w^hen 
we  stand  in  the  middle  of  the  walk,  opposite  the  house, 
the  echo  returns  to  the  person  who  spoke. 

Mrs.  B.  Speaking-trumpets  are  constructed  on  the  prin- 
ciple of  the  reflection  of  sound.  The  voice,  instead  of  being 
diffused  in  the  open  air,  is  confined  within  the  trumpet:  and 
the  vibrations  which  spread  and  fall  against  the  sides  of  the 
instrument,  are  reflected  according  to  the  angle  of  inci- 
dence, and  fall  into  the  direction  of  the  vibrations  which 
proceed  straight  forwards.  The  whole  of  the  vibrations 
are  thus  collected  into  a  focus  ;  and  if  the  ear  be  situated 
in  or  near  that  spot,  the  sound   is  prodigiously  incrr^as^^d. 

■^37.     What  effect  has  the  direction  of  the  wind  on  the  \  el  city 

of  sound  ? 738.     To  what  practical  pur|30se  can  we  ap?)ly  the 

uniform  velocity  of  sound  ? 739.     How  is  the  sound  of  an  echo 

.  produced  ? 740.      On  what  principle  are  speaking-trumpets 

constructed  ? 


ON  WIND  AND  SOUND.  181 

Figure  7,  plate  XIV.  will  give  you  a  clearer  idea  of  the 
speaking-trumpet  :  the  reflected  rays  are  distin^uibhed 
from  those  of  incidence,  by  being  dotted ;  and  they  are 
brought  to  a  focus  at  F.  The  trumpet  used  by  deaf  per- 
sons acts  on  the  same  principle  ;  but  as  the  voice  enters 
the  trumpet  at  the  large  instead  of  the  small  end  of  the 
instrument,  it  is  not  so  much  confined,  nor  the  sound  so 
much  increased. 

Emily,  Are  the  trumpets  used  as  musical  instruments 
also  constructed  on  this  principle  1 

Mrs,  B,  So  far  as  their  form  tends  to  increase  the 
sound,  they  are  ;  but,  as  a  musical  instrument,  the  trum- 
pet becomes  itself  the  sonorous  body,  which  is  made  to 
vibrate  by  blowing  into  it,  and  communicates  its  vibrations 
to  the  air. 

I  will  attempt  to  give  you  in  a  few  words,  some  notion 
x)f  the  nature  of  musical  sounds,  which  as  you  are  fond  of 
musick  must  be  interesting  to  you. 

If  a  sonorous  body  be  struck  in  such  a  manner,  that  its 
vibrations  are  all  performed  in  regular  times,  the  vibra^ 
tions  of  the  air  will  correspond  with  them ;  and  striking 
in  the  same  regular  manner  on  the  drum  of  the  ear,  will 
produce  the  same  uniform  sensation  on  the  auditory  nerve 
and  excite  the  same  uniform  idea  in  the  mind;  or,  in 
other  words,  we  shall  hear  one  musical  tone. 

But  if  the  vibrations  of  the  sonorous  body  are  irregular, 
there  will  necessarily  follow  a  confusion  of  aerial  vibra- 
tions;  for  a  second  vibration  may  commence  before  the 
first  is  finished,  meet  it  half  way  on  its  return,  interrupt 
it  in  its  course,  and  produce  harsh  jarring  sounds  which 
are  called  discords, 

Kmily,  But  each  set  of  these  irregular  vibrations,  if 
repeated  at  equal  intervals,  v/ould,  I  suppose,  produce  a 
musical  tone.  It  is  only  their  irregular  succession  which 
makes  them  interfere,  and  occasions  discord. 


741.     What   does  Figure  7,  Plate    XIV.    represent  ? 743. 

Where  must  the  ear  be  situated  in  regard  to  the  speaking-trumpet 
so  as  to  receive  an  increased  sound  ? 743.  How  do  the  speak- 
ing-trumpets used  by  deaf  persons  differ  from  that  in  the  figure  ? 
744.  How  far  is  a  trumpet  used  for  a  musical  instrument  con- 
structed on  the  above  principle  .'' 745.     How  must  a  sonorous 

body  be  struck  so  that  its  vibrations  produce  in  the  mind  the  same 
uniform  idea,  or  one  musical  tone  ? 746.     How  are  harsh  jar- 
ring sounds  or  discords  produced  .'* 
16 


182  ON  WIND  AND  SOUND. 

Mrs.  JB.  Certainly.  The  quicker  a  sonorous  body  vi- 
brates, the  more  acute,  or  sharp,  is  the  sound  produced. 

Caroline,  But  if  I  strike  any  one  note  of  the  piano- 
forte repeatedly,  whether  quickly  or  slowly,  it  always  gives 
the  same  tone. 

Mrs.  B.  Because  the  vibrations  of  the  same  string,  at 
the  same  degree  of  tension,  are  always  of  a  similar  dura- 
tion. The  quickness  or  slowness  of  the  vibrations  relate 
to  the  single  tones,  not  to  the  various  sounds  which  they 
may  compose  by  succeeding  each  other.  Striking  the 
note  in  quick  succession,  produces  a  more  frequent  repe- 
tition of  the  tone,  but  does  not  increase  the  velocity  of  the 
vibrations  of  the  string. 

The  duration  of  the  vibrations  of  strings  or  chords  de- 
pends upon  their  length,  their  thickness,  or  weight,  and 
their  degree  of  tension  :  thus,  you  find,  the  low  bass  notes 
are  produced  by  long,  thick,  loose  strings  ;  and  the  high 
treble  notes  by  short,  small,  and  tight  strings. 

Caroline.  Then  the  different  length  and  size  of  the 
strings  of  musical  instruments,  serve  to  vary  the  duration 
of  the  vibrations,  and  consequently,  the  acuteness  of  gra- 
vity of  the  notes  1 

Mrs.  B.  Yes.  Among  the  variety  of  tones,  there  are 
some  which,  sounded  together,  please  the  ear,  producing 
\vhat  we  call  harmony,  or  concord.  This  arises  from  the 
agreement  of  the  vibrations  of  the  two  sonorous  bodies  ; 
so  that  some  of  the  vibrations  of  each  strike  upon  the  ear 
at  the  same  time.  Thus,  if  the  vibrations  of  two  strings 
are  performed  in  equal  times,  the  same  tone  is  produced 
by  both,  and  they  are  said  to  be  in  unison. 

Emily.  Now^,  then,  I  understand  why,  when  I  tune 
my  harp  in  unison  with  the  piano-forte,  I  draw  the  strings 
tighter  if  it  is  too  low,  or  loosen  them  if  it  is  at  too  high  a 
pitch  ;  it  is  in  order  to  bring  them  to  vibrate,  in  equal 
times,  with  the  strings  of  the  piano- forte. 

Mrs.  B.  But  concord,  you  know,  is  not  confined  to 
unison  ;  for  two  different  tones  harmonize  in  a  variety  of 
cases.  If  the  vibrations  of  one  string  (or  sonorous  body 
whatever)  vibrate  in  double  the  time  of  another,  the  se- 
eond  vibration  of  the  latter  will  strike  upon  the  ear  at  the 

747.     On  what  does  the   acuteness   or  sharpness  of  a  musical 

sound  depend  ? 748.     On  what  does  the  duration  of  vibrations 

of  strings  or   chords  in   musical    instruments  depend  .'* 749. 

How  is  harmony  or  concord  in  sounds  produced  .'' 750.     Howie 

an  octave  concord  produced  ^ 


ON  OPTICKS.  1S3 

same  instant  as  the  first  vibration  of  the  former  ;  and  this 
is  the  concord  of  an  octave. 

If  the  vibrations  of  two  strings  are  as  two  to  three,  the 
second  vibration  of  the  first  corresponds  with  the  third  vi- 
bration of  the  latter,  producing  the  harmony  called  a  fifth. 

Caroline,  So,  then,  when  I  strike  the  key-note  with 
its  fifth,  I  hear  every  second  vibration  of  one,  and  every 
third  of  the  other  at  the  same  time  ? 

Mrs,  B.  Yes;  and  the  key-note  struck  with  the 
fourth  is  likewise  a  concord,  because  the  vibrations  are  av^ 
three  to  four.  The  vibrations  of  a  major  third  with*  the 
key-note,  are  as  four  to  five  ;  and  those  of  a  minor  third,  as 
five  to  six. 

There  are  other  tones  which,  though  they  cannot  be 
struck  together  without  producing  discord,  if  struck  suc- 
cessively, give  us  the  pleasure  which  is  called  melody. 
Upon  these  general  principles  the  science  of  musick  i? 
founded  ;  but  I  am  not  sufficiently  acquainted  with  it  to 
enter  any  further  into  it.* 

We  shall  now,  therefore,  take  leave  of  the  subject  of 
sound ;  and,  at  our  next  interview,  enter  upon  that  of  op- 
ticks,  in  which  we  shall  consider  the  nature  of  vision, 
light,  and  colours. 

751.     How  is  that  species  of  harmony,  called  a  fifth,  produced  f 


CONVERSATION  XIV. 

ON  OPTICKS. 


Of  Luminous,  Transparent,  and  Opaque  Bodies;  Of 
the  Radiation  of  Light ;  Of  Shadows ;  Of  the  Reflect 
Hon  of  Light ;  Opaque  Bodies  seen  only  by  Refected 
Light ;  Vision  explained ;  Camera  Ohscura ;  Image 
of  Objects  on  the  Retina, 


CAROLINE. 


I  LONG  to  begin  our  lesson  to-day,  Mrs.  B.,  for  I  ex- 
pect that  it  will  be  very  entertaining. 

*  When  musick  is  made  by  the  use  of  strings,  the  air  is  struck 
by  the  body,  and  the  sound  is  excited  by  the  vibrations  :  when  it  is 
made  by  pipes,  the  body  is  struck  by  the  air  ;  but  as  action  and  re- 
action are  equal,  the  effect  is  liie  same  in  both  cases. 


184  Of^   OPTICKS. 

Mrs,  B,  Opticks  is  certainly  one  of  the  most  interesi- 
ing  branches  of  Natural  Philosophy,  but  not  one  of  the 
easiest  to  understand  ;  I  must  therefore  beg  that  you  will 
give  me  the  whole  of  your  attention. 

I  shall  first  inquire,  whether  you  comprehend  the  mean- 
ing of  a  luminous  hodi/,  an  opaque  hody,  and  a  transparent 
body, 

Caroline,  A  kiminous  body  is  one  that  shines  ;  an 
opaque  .... 

Mrs,  B,  Do  not  proceed  to  the  second,  until  we  have 
agreed  upon  the  definition  of  the  first.  All  bodies  that 
shine  are  not  luminous  ;  for  a  luminous  body  is  one  that 
shines  by  its  own  light,  as  the  sun,  the  fire,  a  candle,  &.C.* 

Emily,  Polished  metal,  then,  when  it  shines  with  so 
much  brilliancy,  is  not  a  luminous  body  ? 

Mrs,  B,  No,  for  it  w  ould  be  dark  if  it  did  not  receive 
light  from  a  luminous  body ;  it  belongs,  therefore,  to  the 
class  of  opaque  or  dark  bodies,  which  comprehend  all 
such  as  are  neither  luminous  nor  will  admit  the  light  to 
pass  through  them. 

Emily,  And  transparent  bodies,  are  those  which  ad- 
mit the  light  to  pass  through  them  ;  such  as  glass  and 
water. 

Mrs.  B,  You  are  right.  Transparent  or  pellucid 
bodies  are  frequently  called  mediums  ;  and  the  rays  of 


*  The  direct  light  of  the  sun  is  calculated  to  be  equal  to  that  of 
6560  candles,  placed  at  the  distance  of  one  foot  from  the  object  ; 
and  that  of  the  moon  to  the  light  of  one  candle  at  TJ  feet  distance  \ 
of  Jupiter  at  1620  feet,  and  of  Venus  at  421  feet.  Sir  Isaac  New- 
ton supposed  rays  of  light  to  consist  of  exceedingly  small  particles, 
infinitely  smaller  than  sand,  moving  from  luminous  bodies  ;  but 
later  writers  suppose  them  to  consist  of  the  undulations  of  an  elas- 
tick  medium,  which  fills  all  space,  and  which  produces  the  sensa- 
tion of  light  to  the  eye,  just  as  the  vibrations  of  tlie  air  prodiice  the 
sensation  of  sound  to  the  ear.  ' 


752.     What  is  the  science  called  that  treats  of  vision  r 753^ 

What  is  a  luminous  body  ? 754.      To  ichat  is  the  direct  light  of 

the  sun  calculated  to  be  equal .?— — 755.     To  ichat  is  the  light  of 
the  moon — of  Jupiter — ajid  of  Venus,  respectively  calculated  to  he 

%qual  ? 756.     What  was  Sir  Isaac  Kewton's  opinion  concerning 

the  nature  of  li^rht  ? 757.      What  is  a  modern   opinion  ? 

758.     What' are^  opaque  bodies  .=• 750.     What  are  transparent 

bodies  ? 760      AVhat  are  transparent  bodies  frequently  called  : 


O^  OPTICKS.  185 

light  which  pass  through  them,  are  said  to  be  transmitted 
by  them. 

Light,  when  emanated  from  the  sun,  or  any  other  lumi- 
nous body,  is  projected  forwards  in  straight  lines  in  every 
possible  direction  ;  so  that  the  luminous  body  is  not  only 
the  general  centre  from  whence  all  the  rays  proceed,  but 
every  point  of  it  may  be  considered  as  a  centre  which  ra- 
diates light  in  every  direction.     (Fig.  1.  plate  XV.) 

Emily,  But  do  not  the  rays  which  are  projected  in 
different  directions,  and  cross  each  other,  interfere,  and 
impede  each  other's  course  ? 

Mrs.  B,  Not  at  all.  The  particles  of  light  are  so  ex- 
tremely minute,  that  they  are  never  known  to  interfere 
with  each  other.  A  ray  of  light  is  a  single  line  of  light 
projected  from  a  luminous  body  ;  and  a  pencil  of  rays,  is 
a  collection  of  rays,  proceeding  from  any  one  point  of  a 
luminous  body,  as  fig.  2. 

Caroline.  Is  light  then  a  substance  composed  of  par- 
ticles like  other  bodies  1 

Mrs.  B.  This  is  a  disputed  point  upon  which  I  can- 
not pretend  to  decide.  In  some  respects,  light  is  obedi- 
ent to  the  laws  which  govern  bodies  ;  in  others  it  appears 
to  be  independent  of  them :  thus,  though  its  course  is 
guided  by  the  laws  of  motion,,  it  does  not  seem  to  be  in- 
fluenced by  those  of  gravity.  It  has  never  been  disco- 
vered to  have  weight,  though  a  variety  of  interesting  ex- 
periments have  been  made  with  a  view  of  ascertaining 
that  point  ;  but  we  are  so  ignorant  of  the  intimate  nature 
of  light,  that  an  attempt  to  investigate  it  would  lead  us 
into  a  labyrinth  of  perplexity,  if  not  of  errour  ;  we  shall 
therefore  confine  our  attention  to  those  properties  of  light 
which  are  well  ascertained. 

Let  us  return  to  the  examination  of  the  effects  of  the 
radiation  of  light  from  a  luminous  body.  Since  the  rays 
of  light  are  projected  in  straight  lines,  when  they  meet 

761.     In  what  manner  is  light  produced  from  luminous  bodies  ? 

762.     What  is  the  reason  th.it  the  progress  of  rays  of  light  is 

not  impeded  by  crossing  each  other  ? 763.     What  is  a  ray  of 

light  ^ 764.  What  is  a  pencil  of  rays  ? 765.  Is  light  a  sub- 
stance composed  of  particles  of  matter,  like  other  bodies  ■' 766. 

In  what  respect  is  it  subject  to  the  laws  of  matter  ? 767.     In 

what  respect  is  it  not  subject  to  the    laws  of  matter  '* 766. 

What  is  the  consequence  when  rays  of  light  fall  upon  an  opaque 
body ' 

16* 


186  ON  OPTICKS. 

with  an  opaque  body  through  which  they  are  unable  to 
pass,  they  are  stopped  short  in  their  course  ;  for  they  can- 
not move  iii  a  curve  line  round  the  body. 

Caroline,  No,  certainly ;  for  it  would  require  some 
other  force  besides  that  of  projection,  to  produce  motion 
in  a  curve  line. 

Mrs.  B,  The  interruption  of  the  rays  of  light,  by  the 
opaque  body,  produces,  therefore,  darkness  on  the  oppo- 
site side  of  it ;  and  if  this  darkness  fall  upon  a  wall,  a 
sheet  of  paper,  or  any  object  \;  hatever,  it  forms  a  shadow. 

Emily,  A  shadow  then  is  nothing  more  than  darkness 
produced  by  the  intervention  of  an  opaque  body,  which 
prevents  the  rays  of  light  from  reaching  an  object  behind 
the  opaque  body. 

Caroline,  Why  then  are  shadows  of  different  degrees 
of  darkness  :  for  I  should  have  supposed,  from  your  defi- 
nition of  a  shadow,  that  it  would  have  been  perfectly 
black  ? 

Mrs,  B,  It  frequently  happens  that  a  shadow  is  pro- 
duced by  an  opaque  body  interrupting  the  course  of  the 
rays  from  one  luminous  body,  while  light  from  another 
reaches  the  space  v/here  the  shadow  is  formed,  in  which 
case  the  shadow  is  proportionally  fainter.  This  happens 
if  the  opaque  body  be  lighted  by  two  candles  :  if  you  ex- 
tinguish one  of  them,  the  shadow  will  be  both  deeper  and 
more  distinct. 

Caroline,     But  yet  it  will  not  be  perfectly  dark. 

Mrs,  B,  Because  it  is  still  slightly  illumined  by  light 
reflected  from  the  walls  of  the  room,  and  other  surround- 
ing objects. 

You  must  observe,  also,  that  when  a  shadow  is  pro- 
duced by  the  interruption  of  rays  from  a  single  luminous 
body,  the  darkness  is  proportional  to  the  intensity  of  the 
light. 

Emily,  I  should  have  supposed  the  contrary ;  for  as 
ihe  light  reflected  from  surrounding  objects  on  the  sha- 
dow, must  be  in  proportion  to  the  intensity  of  the  light,  the 
stronger  the  light,  the  more  the  shadow  will  be  illumined. 


769.  What  does  this  interruption  produce  in  regard  to  the  body  ? 
770.  What  is  a  shadow  ? 771.  Why  are  shadows  of  diffe- 
rent degrees  of  darkness  ? 772.     When  a  shadow  is  produced 

by  the  interruption  of  rays  of  light  from  a  single  opaque  body,  tc 
what  is  the  darkness  of  the  shadow  proportional  ? 


ON  OPTICKS.  187 

3frs,  B,  Your  remark  is  perfectly  just ;  but  as  we  have 
no  means  of  estimating  the  degrees  of  light  and  of  dark- 
ness but  by  comparison,  the  strongest  light  will  appear  to 
produce  the  deepest  shadow.  Hence  a  total  eclipse  of  the 
sun  occasions  a  more  sensible  darkness  than  midnight, 
as  it  is  immediately  contrasted  with  the  strong  light  of 
noon-day. 

Caroline,  The  re-appearance  of  the  sun  after  an 
eclipse,  must,  by  the  same  contrast,  be  remarkably  brilliant. 

Mrs.  B,  Certainly.  There  are  several  things  to  be 
observed  in  regard  to  the  form  and  extent  of  shadows. 
If  the  luminous  body  A  (fig.  3.)  is  larger  than  tlie  opaque 
body  B,  the  shadow  will  gradually  diminish  in  size,  till  it 
terminate  in  a  point. 

Caroline.  This  is  the  case  with  the  shadows  of  the 
earth  and  the  moon,  as  the  sun  which  illumines  them,  is 
larger  than  either  of  those  bodies.  And  vvliy  is  it  not  the 
case  with  the  shadows  of  terrestrial  objects,  which  are 
equally  illumined  by  the  sun  ?  but  their  shadows,  far  from 
diminishing,  are  always  larger  than  the  object,  and  in- 
crease with  the  distance  from  it. 

Mrs.  B.  In  estimating  the  effect  of  shadows,  we  must 
consider  the  apparent  not  the  real  dimensions  of  the  lu- 
minous body  ;  and  in  this  point  of  view,  the  sun  is  a  small 
object  compared  with  the  generality  of  the  terrestrial  bo- 
dies which  it  illumines  :  and  when  the  luminous  body  is 
less  than  the  opaque  body,  the  shadow  will  increase  with 
the  distance  to  infinity.  All  objects,  therefore,  which  are 
apparently  larger  than  the  sun,  cast  a  magnified  shadow. 
This  will  be  best  exemplified,  by  observing  the  shadow  of 
an  object  lighted  by  a  candle. 

Emily.  I  have  often  noticed,  that  the  shadow  of  my 
figure  against  the  wall,  grows  larger  as  it  is  more  distant 
from  me,  which  is  owing,  no  doubt,  to  the  candle  that 
shines  on  me  being  much  smaller  than  myself  ? 

Mrs.  B.  Yes.  The  shadow  of  a  figure  A,  (fig.  4.) 
varies  in  size,  according  to  the  distance  of  the  several  sur- 
faces B  C  D  E,  on  which  it  is  described. 

773.  Why  does  a  total  eclipse  of  the  sun  occasion  a  more  sen- 
sible darkness  than  midnight  ? 774.      What  will  be  the  form  of 

the  shadow  when  a  luminous  body  is  larger  than  the  opaque  body 

upon  which  it  shines  ? 775.     And  why  is  it  not  the  case  with 

shadows  of  terrestrial  objects,  which  are  illumined  by  the  sun  ? 

776.     When  the  luminous  body  is  less  than  the  opaque  body,  how 
does  th^  shadow  increase  ? 777.     Which  figure  illustrates  this  ^ 


188  ON  OPTICKS. 

Caroline,  I  have  observed,  that  two  candles  produce 
two  shadows  from  the  same  object ;  whilst  it  would  ap- 
pear from  what  you  said,  that  they  should  rather  produce 
only  half  a  shadow,  that  is  to  say,  a  very  faint  one. 

Mrs,  B,  The  number  of  lights  (indifferent  directions) 
while  it  decreases  the  intensity  of  the  shadow,  increases 
their  number,  which  always  corresponds  with  that  of  the 
lights  ;  for  each  light  makes  the  opaque  body  cast  a  diffe- 
rent shadow,  as  illustrated  by  fig.  5.  It  represents  a  ball 
A,  lighted  by  three  candles  B,  C,  D,  and  you  observe  the 
light  B  produces  the  shadow  b,  the  light  C  the  shadow  c, 
and  the  light  D  the  shadow  d, 

Emily.  I  think  we  now  understand  the  nature  of 
shadows  very  well  ;  but  pray  what  becomes  of  the  rays  of 
light  which  opaque  bodies  arrest  in  their  course,  and  the 
interruption  of  which  is  the  occasion  of  shadows  ? 

Mrs,  B,  Your  question  leads  to  a  very  important  pro- 
perty of  light.  Reflection,  When  rays  of  light  encounter 
an  opaque  body,  which  they  cannot  traverse,  part  of  them 
are  absorbed  by  it,  and  part  are  reflected,  and  rebound 
just  as  an  elastick  ball  which  is  struck  against  a  wall. 

Emily,  And  is  light  in  its  reflection  governed  by  the 
same  laws  as  soli«l  elastick  bodies  ? 

Mrs,  B,  Exactly.  If  a  ray  of  light  fall  perpendicu- 
larly on  an  opaque  body,  it  is  reflected  back  in  the  same 
line,  towards  the  point  whence  it  proceeded.  If  it  fall  ob- 
liquely, it  is  reflected  obliquely,  but  in  the  opposite  direc- 
tion ;  the  angle  of  incidence  being  equal  to  the  angle  of 
reflection.     You  recollect  that  law  in  mechanicks  ? 

Emily,     Oh  yes,  perfectly. 

Mrs,  B,  If  you  will  close  the  shutters,  we  shall  ad- 
mit a  ray  of  the  sun's  light  through  a  very  small  aperture, 
and  I  can  show  you  how  it  is  reflected.  I  now  hold  this 
mirror,  so  that  the  ray  shall  fall  perpendicularly  upon  it. 


778.     How  may  more  shadows  than  one  be  produced  by  a  single 

opaque  body  ? 779.     By  which  figure  is  this  illustrated  ? 

780.     What  is  meant  by  the  reflection  of  light  ? 781.     Is  all  the 

light  that  fails  upon  an  opaque  body  reflected  ? 782.     By  what 

laws  is  the  reflection  of  light  governed  ? ^783.     If  a  ray  of  light 

fall  upon  an  opaque  body  perpendicularly,  how  will  it  be  reflected  ? 

784.     How  will  it  be  reflected  if  it  fall  upon  an  opaque  body 

obliquely  ? ^785.     How  does  the  angle  of  incidence  compare 

with  the  angle  of  reflection  ? 


ON  OPTICKS.  181^ 

Caroline,  I  see  the  ray  which  falls  upon  the  mirror, 
but  not  that  which  is  reflected  by  it. 

Mrs,  B.  Because  its  reflection  is  directly  retrograde^ 
The  ray  of  incidence  and  that  of  reflection  both  being  in 
the  same  line,  though  in  opposite  directions,  are  confound- 
ed together. 

Emily.  The  ray  then  which  appears  to  us  single,  is 
really  double,  and  is  composed  of  the  incident  ray  pro- 
ceeding to  the  mirror,  and  of  the  reflected  ray  returning 
from  the  mirror. 

Mrs,  B.  Exactly  so.  We  shall  now  separate  them 
by  holding  the  mirror  M,  (fig.  6.)  in  such  a  manner,  that 
the  incident  ray  A  B  shall  fall  obliquely  upon  it — you  see 
the  reflected  ray  B  C,  is  marching  off  in  another  direc- 
tion. If  we  draw  a  line  from  the  point  of  incidence  B, 
perpendicular  to  the  mirror,  it  will  divide  the  angle  of 
incidence  from  the  angle  of  reflection,  and  you  will  see 
that  they  are  equal. 

Emily.  Exactly  ;  and  now  that  you  hold  the  mirror 
so  that  the  ray  falls  more  obliquely  on  it,  it  is  also  reflected 
more  obliquely,  preserving  the  equality  of  the  angles  of 
incidence  and  reflection. 

Mrs.  B.  It  is  by  reflected  rays  only  that  we  see 
opaque  objects.  Luminous  bodies  send  rays  of  light  im- 
mediately to  our  eyes,  but  the  rays  which  they  send  to 
other  bodies  are  invisible  to  us,  and  are  seen  only  when 
they  are  reflected  or  transmitted  by  those  bodies  to  our 
eyes. 

Emily.  But  have  we  not  just  seen  the  ray  of  light  in 
its  passage  from  the  sun  to  the  mirror,  and  its  reflection  ?* 
yet  in  neither  case  were  those  rays  in  a  direction  to  enter 
our  eyes. 

Mrs.  B.  No.  What  you  saw  was  the  light  reflected 
to  your  eyes  by  small  particles  of  dust  floating  in  the  air, 
and  on  which  the  ray  shone  in  its  passage  to  and  from  the 
mirror. 

Caroline.  Yet  I  see  the  sun  shining  on  that  house 
yonder,  as  clearly  as  possible. 

Mrs.  B.  Indeed  you  cannot  see  a  single  ray  which 
passes  from  the  sun  to  the  house  ;  you  see  no  rays  but 

786.  Which  figure  illustrates  the  manner  in  which  li^ht  is  re- 
flected ? 787.     By  what  rays  do   we  see  opaque   bodies  ? 

788.     How  are  we  able  to  see  light  that  falls  upon  an  opaque  body 
and  is  reflected;  but  not  in  a  direction  to  meet  the  eye  r 


100  ON  OPTICKS. 

those  which  enter  your  eyes;  therefore  it  is  the  rays 
which  are  reflected  by  the  house  to  you,  and  not  those 
which  proceed  from  the  sun  to  the  house,  that  are  visible 
to  you. 

Caroline,  Why  then  does  one  side  of  the  house  ap- 
pear to  be  in  sunshine,  and  the  other  in  the  shade  ?  for  if  I 
cannot  see  the  sun  shine  upon  it,  the  w^hole  of  the  house 
should  appear  in  the  shade. 

Mrs,  B,  That  side  of  the  house  which  the  sun  shines 
upon,  reflects  more  vivid  and  luminous  rays  than  the  side 
which  is  in  shadow,  for  the  latter  is  illumined  only  by  rays 
reflected  upon  it  by  other  objects  :  these  rays  are  therefore 
twice  reflected  before  they  reach  your  sight ;  and  as  light 
is  more  or  less  absorbed  by  the  bodies  it  strikes  upon, 
every  time  a  ray  is  reflected  its  intensity  is  diminished. 

Caroline,  Still  I  cannot  reconcile  myself  to  the  idea, 
that  we  do  not  see  the  sun's  rays  shining  on  objects,  but 
only  those  which  objects  reflect  to  us. 

Mrs,  B,  I  do  not,  however,  despair  of  convincing  you 
of  it.  Look  at  that  large  sheet  of  water  ;  can  you  tell  why 
the  sun  appears  to  shine  on  one  part  of  it  only  ? 

Caroline,  No,  indeed  ;  for  the  whole  of  it  is  equally 
exposed  to  the  sun.  This  partial  brilliancy  of  water  has 
often  excited  my  wonder  ;  but  it  has  struck  me  more  par- 
ticularly by  moon-light.  I  have  frequently  observed  a 
vivid  streak  of  moon-shine  on  the  sea,  while  the  rest  of 
the  water  remained  in  deep  obscurity,  and  yet  there  was 
no  apparent  obstacle  to  prevent  the  moon  from  shining  on 
every  part  of  the  water  equally. 

*  Mrs,  B.  By  moon-light  the  effect  is  more  remarkable, 
on  account  of  the  deep  obscurity  of  the  other  parts  of  the 
water  ;  while  by  the  sun's  light  the  effect  is  too  strong  for 
the  eye  to  be  able  to  contemplate  it. 

Caroline,  But  if  the  sun  really  shines  on  every  part  of 
that  sheet  of  water,  why  does  not  every  part  of  it  reflect 
rays  to  my  eyes  1 

Mrs,  B.  The  reflected  rays  are  not  attracted  out  of 
th^ir  natural  course  by  your  eyes.     The  direction  of  a 


789.  What  does  one  side  of  an  opaque  body  appear  to  be  in  the 
sun-shine  and  the  other  in  the  shade,  when  by  not  seeing  the  rays 
that  fall  upon  the   object,  both  sides  of  it  would  appear  shaded  ? 

790.     What  illustration  is  given  to  show  that  we  only  see  the 

reflected  light  which  falls  upon  different  objects  ' 


ON  OPTICKS.  191 

reflected  ray,  you  know,  depends  on  that  of  the  incident 
ray  ;  the  sun's  rays,  therefore,  which  fail  with  various  de- 
grees of  obliquity  upon  the  water,  are  reflected  in  direc- 
tions equally  various  ;  some  of  these  will  meet  your  eyes, 
and  you  will  see  them,  but  those  which  fall  elsewhere  are 
invisible  to  you. 

Caroline,  The  streak  of  sunshine,  then,  which  we 
now  see  upon  the  water,  is  composed  of  those  rays  which 
by  their  reflection  happen  to  fall  upon  my  eyes  ? 

Mrs.  B,     Precisely. 

Emily.  But  is  that  side  of  the  house  yonder,  which 
appears  to  be  in  shadow,  really  illumined  by  the  sun,  and 
its  rays  reflected  another  way. 

Mrs.  B.  No ;  that  is  a  different  case  from  the  sheet 
of  water.  That  side  of  the  house  is  really  in  shadow  ;  it 
is  the  west  side,  which  the  sun  cannot  shine  upon  till  the 
afternoon. 

Enitly.  Those  objects,  then,  which  are  illumined  by 
reflected  rays,  and  those  which  receive  direct  rays  from  the 
sun,  but  which  do  not  reflect  those  rays  towards  us,  ap- 
pear equally  in  shadow  ? 

Mrs.  B.  Certainly ;  for  we  see  them  both  illumined 
by  reflected  rays.  That  part  of  the  sheet  of  water,  over 
which  the  trees  cast  a  shadow,  by  what  light  do  you  see 
it? 

Emily,  Since  it  is  not  by  the  sun's  direct  rays,  it  must 
be  by  those  reflected  on  it  from  other  objects,  and  which 
it  again  reflects  to  us. 

Caroline.  But  if  we  all  see  terrestrial  objects  by  re- 
flected light,  (as  we  do  the  moon,)  why  do  they  appear  so 
bright  and  luminous  ?  I  should  have  supposed  that  re- 
flected rays  would  have  been  dull  and  faint,  like  those  of 
the  moon. 

Mrs.  B.  The  moon  reflects  the  sun's  light  with  as 
much  vividness  as  any  terrestrial  object.  If  you  look  at 
it  on  a  clear  night,  it  will  appear  as  bright  as  a  sheet  of 
water,  the  walls  of  a  house,  or  any  object  seen  by  day-light 
and  on  which  the  sun  shines.  The  rays  of  the  moon 
are  doubtless  feeble,  when  compared  with  those  of  the 

791.     Why  is  it  that  the  whole  surface  of  water  on  which  the 

sun  or  moon  shines  does  not  appear  illumined  ? 79'2.  How  does 

the  case  of  the  sheet  of  water  named,  differ  from  that  of  the  house 

on  which  the  sun  shines? 793.     How  are  we  enabled  to  see  thp 

moon  ? 


1^  ON  OPTICKS. 

sun ;  but  that  would  not  be  a  fair  comparison,  lor  the  for- 
mer are  incident,  the  latter  reflected  rays. 

Caroline,  True  ;  and  when  we  see  terrestrial  objects 
by  moonlight,  the  light  has  been  twice  reflected,  and  is 
consequently  proportionally  fainter. 

Mrs,  B,  In  traversing  the  atmosphere,  the  rays,  both 
of  the  sun  and  moon,  lose  some  of  their  light.  For  though 
the  pure  air  is  a  transparent  medium,  which  transmits  the 
rays  of  light  freely,  we  have  observed,  that  near  the  sur- 
face of  the  earth  it  is  loaded  with  vapours  and  exhalations, 
by  which  some  portion  of  them  are  absorbed. 

Caroline.  I  have  often  noticed  that  an  object  on  the 
summit  of  a  hill  appears  more  distinct  than  one  at  an 
equal  distance  in  a  valley,  or  on  a  plain  ;  which  is  owing, 
I  suppose,  to  the  air  being  more  free  from  vapours  in  an 
elevated  situation,  and  the  reflected  rays  being  conse- 
quently brighter. 

Mrs,  B,  That  may  have  some  sensible  effect ;  but 
when  an  object  on  the  summit  of  a  hill  has  a  back  ground 
of  light  sky,  the  contrast  with  the  object  makes  its  outline 
more  distinct. 

Caroline.  I  now  feel  well  satisfied  that  we  see  opaque 
objects  only  by  reflected  rays  ;  but  I  do  not  understand 
how  these  rays  show  us  the  objects  from  which  they  pro- 
ceed. 

Mrs.  B,  The  rays  of  light  enter  at  the  pupil  of  the 
eye,  and  proceed  to  the  retina,  or  optick  nerve,  which  is 
situated  at  the  back  part  of  the  eye-ball ;  and  there  they 
describe  the  figure,colour,  and  (excepting  size)  form  a  per- 
fect representation  of  the  object  from  which  they  proceed. 
We  shall  again  close  the  shutters,  and  admit  the  light 
through  the  small  aperture,  and  you  will  see  a  picture 
on  the  wall,  opposite  the  aperture,  similar  to  that  which 
is  delineated  on  the  retina  of  the  eye. 

Caroline,  Oh,  how  wonderful  !  there  is  an  exact  pic- 
ture in  miniature  of  the  garden,  the  gardener  at  work,  the 

794.  What  effect  is  produced  on  the  sun  and  moon's  rays  from 
traversing  the  atmosphere  ? 795.  What  is  there  in  the  atmo- 
sphere that  has  a  tendency  to  absorb  the  rays  of  light .' 796. 

Why  is  it  that  objects  on  a  hill  appear  more  distinct  than  at  an 

equal  distance  from  us  in  a  valley  .'' 797.     How  is  it  that  the 

rays  of  light  give  us  an  idea  of  the  objects  from  which  they  pro- 
ceed .' 798.     What  experiment  illustrates  the  manner  in  which 

objects  are  delineated  on  the  retina  of  the  eye  ? 


QK  OPTICKS.  193 

trees  blown  about  by  the  wind*  The  landscape  would  be 
perfect,  if  it  were  not  reversed  ;  the  ground  being  above 
and  the  sky  beneath. 

Mrs,  B,  It  is  not  enough  to  admire,  you  must  under- 
stand this  phenomenon,  which  is  called  a  camera  obscura, 
from  the  necessity  of  darkening  the  room,  in  order  to  ex- 
hibit it. 

This  picture  is  produced  by  the  rays  of  light  reflected 
from  the  various  objects  in  the  garden,  and  which  are  ad- 
mitted through  the  hole  in  the  window  shutter. 

The  rays  from  the  glittering  weathercock  at  the  top  of 
the  alcove  A,  (pi.  XVI.  fig.  1.)  represent  it  in  this  spot  a; 
for  the  weathercock  being  much  higher  than  the  aperture  in 
the  shutter,  only  a  few  of  the  rays,  which  are  reflected  by 
it  in  an  obliquely  descending  direction,  can  find  entrance 
there.  The  rays  of  light,  you  know,  always  move  in 
straight  lines  ;  those,  therefore,  which  enter  the  room  in 
a  descending  direction,  will  continue  their  course  in  the 
same  direction,  and  will,  consequently,  fall  upon  the  low- 
er part  of  the  wall  opposite  the  aperture,  and  represent 
the  weathercock  reversed  in  that  spot,  instead  of  erect  in 
the  uppermost  part  of  the  landscape. 

Emily,  And  the  rays  of  light  from  the  steps  (B)  of  the 
alcove,  in  entering  the  aperture,  ascend,  and  will  describe 
those  steps  in  the  highest  instead  of  the  lowest  part  of  the 
landscape. 

Mrs.  B,  Observe,  too,  that  the  rays  coming  from  the 
alcove,  which  is  to  our  left,  describe  it  on  the  wall  to 
the  right ;  while  those  which  are  reflected  by  the  walnut 
tree  C  D,  to  our  right,  delineate  its  figure  in  the  picture  to 
the  left  c  d.  Thus  the  rays,  coming  in  different  directions, 
and  proceeding  always  in  right  lines,  cross  each  other  at 
their  entrance  through  the  aperture :  those  which  come 
above  proceed  below,  those  from  the  right  go  to  the  left, 
those  from  the  left  towards  the  right ;  thus  every  object 
is  represented  in  the  picture,  as  occupying  a  situation  the 
very  reverse  of  that  which  it  does  in  nature. 

Caroline,    Excepting  the  flower-pot  E  F,  which,  though 


799.     What  is  this  illustration  called  ? 800.     From  what  cir- 
cumstance  does   the    camera  obscura  derive  its  name  ? 801» 

How  would  you  explain   Figure  1,  plate  XVI,  as  illustrating  the 

camera  obscura  ? 802.     Why  do  the  objects  exhibited  by  the 

eamera  obscura  appear  inverted  '' 
17 


194  ON  OPTICKS. 

its  position  is  reversed,  has  not  changed  its  situation  in 
the  landscape. 

Mrs.  B,  The  flower-pot  is  directly  in  front  of  the 
aperture  :  so  that  its  rays  fall  perpendicularly  upon  it,  and 
consequently,  proceed  perpendicularly  to  the  wall,  where 
they  delineate  the  object  directly  behind  the  aperture. 

Emily,  And  is  it  thus  that  the  picture  of  objects  is 
painted  on  the  retina  of  the  eye  ?* 

Mrs,  B,  Precisely.  The  pupil  of  the  eye,  through 
which  the  rays  of  light  enter,  represents  the  aperture  in  the 
window-shutter ;  and  the  image  delineated  on  the  retina, 
is  exactly  similar  to  the  picture  on  the  wall. 

Caroline,  You  do  not  mean  to  say,  that  we  see  only 
the  representation  of  the  object  which  is  painted  on  the 
retina,  and  not  the  object  itself? 

Mrs,  B,  If,  by  sight  you  understand  that  sense  by 
which  the  presence  of  objects  is  perceived  by  the  mind, 
through  the  means  of  the  eyes,  we  certainly  see  only  the 
image  of  those  objects  painted  on  the  retina. 

Caroline,     This  appears  to  me  quite  incredible. 

Mrs.  B.  The  nerves  are  the  only  part  of  our  frame 
capable  of  sensation  ;  they  appear,  therefore,  to  be  the 
instruments  which  the  mind  employs  in  its  perceptions  ; 
for  a  sensation  always  conveys  an  idea  to  the  mind.  Now 
it  is  known,  that  our  nerves  can  be  affected  only  by 
contact ;  and  for  ^his  reason  the  organs  of  sense  cannot 
act  at  a  distance ;  for  instance,  we  are  capable  of  smell- 
ing only  particles  which  are  actually  in  contact  with  the 
nerves  of  the  nose.  We  have  already  observed,  that 
the  odour  of  a  flower  consists  in  effluvia,  composed  of 
very  minute  particles,  which  penetrate  the  nostrils,  and 


**  Take  off  the  sclerotica  from  the  back  part  of  the  eye  of  an  ox, 
or  other  animal,  and  place  the  eye  in  the  hole  of  the  window-shut- 
ter of  a  dark  room,  with  its  fore  part  towards  the  external  objects  ; 
a  person  in  the  room  will,  through  the  transparent  coat,  see  the 
inverted  image  painted  upon  the  retina. 


803.  What  part  of  the  eye  is  represented  by  the  aperture  in  the 
window-shutter  ? — —804.  And  to  wliat  is  the  picture  on  the  wall 
in  the  camera  obscura  similar  ? 805.  Do  we  receive  the  sensa- 
tion of  objects  before  us,  from  the  images  formed  on  the  retina  of 
the  eye,  or  direct  from  the  objects  themselves  ?—t — 806.  How  is 
*n  i^ea  of  visible  objects  conveyed  to  the  mind  ^ 


ON  OPTICKS.  *\  196 

Strike  upon  the  olfactory  nerves,  which  instantly  convey 
the  idea  of  smell  to  the  mind. 

Emily,  And  sound,  though  it  is  said  to  be  heard  at  a 
distance,  is,  in  fact,  heard  only  when  the  vibrations  of  the 
air,  which  convey  it  to  our  ears,  strike  upon  the  auditory 
nerve. 

Caroline.  There  is  no  explanation  required  to  prove 
that  the  senses  of  feeling  and  of  tasting  are  excited  only 
by  contact. 

Mrs,  B,  And  I  hope  to  convince  you  that  the  sense 
of  sight  is  so  likewise.  The  nerves,  which  constitute  the 
sense  of  sight,  are  not  different  in  their  nature  from  those 
of  the  other  organs  ;  they  are  merely  instruments  which 
convey  ideas  to  the  mind,  and  can  be  affected  only  on 
contact.  Now  since  real  objects  cannot  be  brought  to 
touch  the  optick  nerve,  the  image  of  them  is  conveyed 
thither  by  the  rays  of  light  proceeding  from  real  objects, 
which  actually  strike  upon  the  optick  nerve,  and  form  that 
image  which  the  mind  perceives. 

Caroline,  While  I  listen  to  your  reasoning,  I  feel  con- 
vinced ;  but  when  I  look  upon  the  objects  around,  and 
think  that  I  do  not  see  them,  but  merely  their  image 
painted  in  my  eyes,  my  belief  is  again  staggered.  I  can- 
not reconcile  myself  to  the  idea,  that  I  do  not  really  see 
this  book  which  I  hold  in  my  hand,  nor  the  words  which 
I  read  in  it. 

Mrs,  B.  Did  it  ever  occur  to  you  as  extraordinary, 
that  you  never  beheld  your  own  face. 

Caroline.  No  ;  because  I  so  frequently  see  an  exact 
representation  of  it  in  the  looking-glass. 

Mrs,  B,  You  see  a  far  more  exact  representation  of 
objects  on  the  retina  of  your  eye  :  it  is  a  much  more  per- 
fect mirror  than  any  made  by  art. 

Emily,  But  is  it  possible,  that  the  extensive  landscape 
which  I  now  behold  from  the  window,  should  be  repre- 
sented on  so  small  a  space  as  the  retina  of  the  eye  ? 

Mrs,  B,  It  would  be  impossible  for  art  to  paint  ^ 
small  and  distinct  a  miniature ;  but  nature  works  with  a 
surer  hand  and  a  more  delicate  pencil.  That  power, 
which  forms  the  feathers  of  the  butterfly,  and  the  flowerets 
of  the  daisy,  can  alone  portray  so  admirable  and  perfect 

607.  How  may  the  nerves  which  constitute  the  sense  of  sight 
be  considered  ? 


196  ON  OPTICKS. 

a  miniature  as  that  which  is  represented  on  the  retina  of 
the  eye. 

Caroline.  But,  Mrs.  B.,  if  we  see  only  the  image  of 
objects,  why  do  we  not  see  them  reversed,  as  you  showed 
us  they  were,  in  the  camera  obscura  ?  Is  not  that  a  strong 
argument  against  your  theory  ? 

Mrs.  B.  Not  an  unanswerable  one,  I  hope.  The 
image  on  the  retina,  it  is  true,  is  reversed,  like  that  in  the 
camera  obscura ;  as  the  rays,  unless  from  a  very  small 
object,  intersect  each  other  on  entering  the  pupil,  in  the 
same  manner  as  they  do  on  entering  the  camera  obscura. 
The  scene,  however,  does  not  excite  the  idea  of  being  in- 
verted, because  we  always  see  an  object  in  the  direction 
of  the  rays  which  it  sends  to  us. 

Emily.     I  confess  I  do  not  understand  that. 

Mrs.  B.  It  is,  I  think,  a  difficult  point  to  explain 
clearly.  A  ray  which  comes  from  the  upper  part  of  an  ob- 
ject describes  the  image  on  the  lower  part  of  the  retina  ^ 
but  experience  having  taught  us  that  the  direction  of  that 
ray  is  from  above,  we  consider  that  part  of  the  object  it 
represents  as  uppermost.  The  rays  proceeding  from  the 
lower  part  of  an  object  fall  upon  the  upper  part  of  the  re- 
tina ;  but  as  we  know  their  direction  to  be  from  below, 
we  see  that  part  of  the  object  they  describe  as  the  lowest. 

Caroline.  When  I  want  to  see  an  object  above  me,  I 
look  up  ;  when  an  object  below  me,  I  look  down.  Does 
not  this  prove  that  I  see  the  objects  themselves  ?  for  if  I 
beheld  only  the  image,  there  would  be  no  necessity  for 
looking  up  or  down,  according  as  the  object  was  higher 
or  lower  than  myself 

Mrs.  B.  I  beg  your  pardon.  When  you  look  up  to 
an  elevated  object,  it  is  in  order  that  the  rays  reflected 
from  it  should  fall  upon  the  retina  of  your  eyes  ;  but  the 
very  circumstance  of  directing  your  eyes  upwards  con- 
vinces you  that  the  object  is  elevated,  and  teaches  you  to 
consider  as  uppermost  the  image  it  forms  on  the  retina, 
though  it  is,  in  fact,  represented  in  the  lowest  part  of  it. 

When  you  look  down  upon  an  object,  you  draw  your 
conclusion  from  a  similar  reasoning  ;  it  is  thus  that  we 
see  all  objects  in  the  direction  of  the  rays  which  reach 
our  eyes. 

808.  If  objects  are  seen  only  by  their  pictures  on  the  retina  of 
ihe  eye,  why  do  they  not  appear  reversed,  as  in  the  camera  obsctt- 
ra?  * 


ON  THE  ANGLE  OF  VISION.  197 

But  I  have  a  further  proof  in  favour  of  what  I  have  ad- 
vanced, which  I  hope  will  remove  your  remaining  doubts  ; 
I  shall,  however,  defer  it  till  our  next  meeting,  as  the  les- 
son has  been  sufficiently  long  to-day. 


CONVERSATION  XV. 

OPTICKS CONTINUED. 

ON   THE  ANGLE  OF    VISION,  AND    THE  REFLECTION   OF 
MIRRORS. 

Angle  of  Vision;  Reflection  of  Plain  Mirrors ;  Reflection 
of  Convex  Mirrors  ;  Reflection  of  Concave  Mirrors, 

CAROLINE. 

Well,  Mrs.  B.,  I  am  very  impatient  to  hear  what  fur- 
ther proofs  you  have  to  offer  in  support  of  your  theory. 
You  must  allow  that  it  was  rather  provoking  to  dismiss  us 
as  you  did  at  our  last  meeting. 

Mrs,  B.  You  press  so  hard  upon  me  with  your  objec- 
tions, that  you  must  give  me  time  to  recruit  my  forces. 
Can  you  tell  me,  Caroline,  why  objects  at  a  distance  ap- 
pear smaller  than  they  really  are  ? 

Caroline.     I  know  no  other  reason  than  their  distance. 

Mrs.  B.  I  do  not  think  I  have  more  cause  to  be  sa- 
tisfied with  your  reasons  than  you  appear  to  be  with  mine. 
We  must  refer  again  to  the  camera  obscura  to  account 
for  this  circumstance  ;  and  you  will  find,  that  the  different 
apparent  dimensions  of  objects  at  different  distances  pro- 
ceed from  our  seeing,  not  the  objects  themselves,  but 
merely  their  image  on  the  retina.  Fig.  I,  plate  XVII. 
represents  a  row  of  trees,  as  viewed  in  the  camera  obscura. 
I  have  expressed  the  direction  of  the  rays,  from  the  ob- 
jects to  the  image,  by  lines.  Now,  observe,  the  ray  which 
comes  from  the  top  of  the  nearest  tree,  and  that  which 
comes  from  the  foot  of  the  same  tree,  meet  at  the  aperture, 

609.    Why  do  objects  appear  smaller  at  a  distance  than  they 

really  are  ? 810.    What  is  an  angle  of  vision  ? 811.     Which 

figure  illustrates  the  angle  of  vision  ? 812.     How  would  you 

explain  that  figure  in  reference  to  the  effect  that  distance  has  op 
the  apparent  size  of  an  object  ? 
17  ♦ 


196  ON  THE  ANGLE  OP  VISION. 

forming  an  angle  of  about  25  degrees ;  this  is  called  the  an- 
gle of  vision,  under  which  we  see  the  tree.  These  rays 
cross  each  other  at  the  aperture,  forming  equal  angles  on 
each  side  of  it,  and  represent  the  tree  inverted  in  the 
camera  obscura.  The  degrees  of  the  image  are  conside- 
rably smaller  than  those  of  the  object,  but  the  proportions 
are  perfectly  preserved. 

Now  let  us  notice  the  upper  and  lower  ray,  from  the 
■K)st  distant  tree ;  they  form  an  angle  of  not  more  than 
twelve  or  fifteen  degrees,  and  an  image  of  proportional 
dimensions.  Thus,  two  objects  of  the  same  size,  as  the 
two  trees  of  the  avenue,  form  figures  of  different  sizes  in 
the  camera  obscura,  according  to  their  distance  ;  or,  in 
other  words,  according  to  the  angle  of  vision  under  whioh 
they  are  seen.     Do  you  understand  this  ? 

Caroline,     Perfectly. 

Mrs*  B.  Then  you  have  onfy  to  suppose  that  the  re- 
presentation in  the  camera  obscura  is  similar  to  that  on 
the  retina. 

Now  since  objects  in  the  same  magnitudes  appear  to 
be  of  different  dimensions,  when  at  different  distances 
from  us,  let  me  ask  you,  which  it  is  that  we  see ;  the 
real  objects,  which  we  know  do  not  vary  in  size,  or  the 
images,  which  we  know  do  vary  according  to  the  angle  ol 
vision  under  which  we  see  them  ? 

Caroline.  I  must  confess,  that  reason  is  in  favour  of 
the  latter.  But  does  that  chair  at  the  further  end  of  the 
room  form  an  image  on  my  retina  much  smaller  than  this 
which  is  close  to  me  ?  they  appear  exactly  of  the  same 
size. 

Mrs,  B,  1  assure  you  they  do  not.  The  experience 
we  acquire  by  the  sense  of  touch  corrects  the  errours  of 
our  sight  with  regard  to  objects  within  our  reach.  You 
are  so  perfectly  convinced  of  the  real  size  of  objects 
which  you  can  handle,  that  you  do  not  attend  to  their 
apparent  difference. 

Does  that  house  appear  to  you  mueh  smaller  than  when 
you  are  close  to  it  ? 

Caroline.     No,  because  it  is  very  near  us. 

Mrs.  B.  And  yet  you  can  see  the  whole  of  it  through 
one  of  the  windows  of  this  room.  The  image  of  the  house, 
on  your  retina,  must,  therefore,  be  smaller  than  that  of 

813     To  what  is  the  size  of  the  angle  of  vision  proportioned  ^ 


ON  THE  ANGLE  OP  VISION.  199 

the  window  through  which  you  see  it.  It  is  your  know- 
ledge of  the  real  size  of  the  house  which  prevents  your 
attending  to  its  apparent  magnitude.  If  you  were  accus- 
tomed to  draw  from  nature,  you  would  be  fully  aware  of  this 
difference. 

Emily.  And  pray,  what  is  the  reason  that,  when  we 
look  up  an  avenue,  the  trees  not  only  appear  smaller  as 
they  are  more  distant,  but  seem  gradually  to  approach 
each  other  till  they  meet  in  a  point  ? 

Mrs.  B.  Not  only  the  trees,  but  the  road  which  sepa- 
rates the  two  rows,  forms  a  small  visual  angle,  in  propor- 
tion as  it  is  more  distant  from  us ;  therefore  the  width  of 
the  road  gradually  diminishes  as  well  as  the  size  of  the 
trees,  till  at  length  the  road  apparently  terminates  in  a: 
point,  at  which  the  trees  seem  to  meet. 

But  this  effect  of  the  angle  of  vision  will  be  more  fully 
illustrated  by  a  little  model  of  an  avenue,  which  I  have 
made  for  that  purpose.  It  consists  of  six  trees,  leading  to 
a  hexagonal  temple,  and  viewed  by  an  eye,  on  the  retina 
of  which  the  picture  of  the  objects  is  delineated. 

I  beg  that  you  will  not  criticise  the  proportions ;  for 
though  the  eye  is  represented  the  size  of  life,  while  the 
trees  are  not  more  than  three  inches  high,  the  dispropor- 
tion does  not  affect  the  principle,  which  the  model  is  in- 
tended to  elucidate. 

Emily.  The  threads  which  pass  from  the  objects 
through  the  pupil  of  the  eye  to  the  retina,  are,  I  suppose, 
to  represent  the  rays  of  light  which  convey  the  image  of 
the  objects  to  the  retina  ? 

Mrs.  B.  Yes.  I  have  been  obliged  to  limit  the  rays 
to  a  very  small  number,  in  order  to  avoid  confusion  ; 
there  are,  you  see,  only  two  from  each  tree. 

Caroline.  But  as  one  is  from  the  summit,  and  the 
other  from  the  foot  of  the  tree,  they  exemplify  the  diffe- 
rent angles  under  which  we  see  objects  at  diflferent  dis- 
tances, better  than  if  there  were  more. 

Mrs.  B.  There  are  seven  rays  proceeding  from  the 
temple,  one  from  the  summit,  and  two  from  each  of  the  an- 
gles that  are  visible  to  the  eye,  as  it  is  situated  ;  from 

814.  Why  are  we  not  deceived  as  to  the  size  of  objects  if  the 
size  of  their  images  on  the  retina  of  the  eye  is  varied  by  the  dis- 
tance the  objects  are  from  us  ? 815.     Why  does  a  road  or  any 

avenue  appear  to  diminish  in  width,  till  at  length  it  apparently 

terminates  in  a  point  ? 816.     What  is  the  reason  that  objects 

viewed  in  front  appear  larger  than  when  viewed  obliq^uely  ^ 


200  ON  THE  ANGLE  OF  VISION. 

these  you  may  form  a  just  idea  of  the  difference  of  the  an- 
gle of  vision  of  objects  viewed  obliquely,  or  in  front  ;  for 
though  the  six  sides  of  the  temple  are  of  equal  dimen- 
sions, that  which  is  opposite  to  the  eye  is  seen  under  a 
much  larger  angle  than  those  which  are  viewed  obliquely. 
It  is  on  this  principle  that  the  laws  of  perspective  are 
founded. 

Emily,  I  am  very  glad  to  know  that,  for  I  have  lately 
begun  to  learn  perspective,  which  appeared  to  me  a  very 
dry  study  ;  but  now  that  I  am  acquainted  with  the  princi- 
ples on  which  it  is  founded,  I  shall  find  it  much  more  in- 
teresting. 

Caroline.  In  drawing  a  view  from  nature,  then,  we 
do  not  copy  the  real  objects,  but  the  image  they  form  on 
the  retina  of  our  eyes  1 

Mrs,  B.  Certainly.  In  sculpture,  we  copy  nature  as 
she  really  exists  ;  in  painting,  we  represent  her  as  she  ap- 
pears to  us.  It  was  on  this  account  that  I  found  it  diffi- 
cult to  explain  by  a  drawing  the  effects  of  the  angle  of 
vision,  and  was  under  the  necessity  of  constructing  a  mo- 
del for  that  purpose. 

Emily.  I  hope  you  will  allow  us  to  keep  this  model 
some  time,  in  order  to  study  it  more  completely,  for  a 
great  deal  may  be  learned  from  it ;  it  illustrates  the  na- 
ture of  the  angle  of  vision,  the  apparent  diminution  of 
distant  objects,  and  the  inversion  of  the  image  on  the  re- 
tina. But  pray,  why  are  the  threads  that  represent  the 
rays  of  light,  coloured,  the  same  as  the  objects  from  which 
they  proceed  1 

Mrs,  B,  That  is  a  question  which  you  must  excuse 
my  answering  at  present,  but  I  promise  to  explain  it  to 
you  in  due  time. 

I  consent  very  willingly  to  your  keeping  the  model,  on 
condition  that  you  will  make  an  imitation  of  it,  on  the 
same  principle,  but  representing  different  objects. 

We  must  now  conclude  the  observations  that  remain  to 
be  made  on  the  angle  of  vision. 

If  an  object,  with  an  ordinary  degree  of  illumination, 
does  not  subtend  an  angle  of  more  than  two  seconds  of  a 

817.     On  what  principle  are  the  laws  of  perspective  founded  ? 
818.     In  drawing  a  picture  of  any  object  what  are  we  to  fol- 
low ? 819.     How  is  nature  to  be  exhibited  in  sculpture  ? 

820.     How  is  it  to  be  represented  in  painting  .^ 821.     "Wbe» 

arc  objects  invisible  ? 


ON  THE  ANGLE  OF  VISION.  201 

degree,  it  is  invisible.  There  are  consequently  two  cases 
in  which  objects  may  be  invisible,  either  if  they  are  too 
small,  or  so  distant  as  to  form  an  angle  less  than  two  se- 
conds of  a  degree. 

In  like  manner,  if  the  velocity  of  a  body  does  not  ex- 
ceed 20  degrees  in  an  hour,  its  motion  is  imperceptible. 

Caroline!  A  very  rapid  motion  may  then  be  imper- 
ceptible, provided  the  distance  of  the  moving  body  is  suffi- 
ciently great. 

Mrs,  B,  Undoubtedly  ;  for  the  greater  its  distance, 
the  smaller  will  be  the  angle  under  which  its  motion  will 
appear  to  the  eye.  It  is  for  this  reason  that  the  motion 
of  the  celestial  bodies  is  invisible,  notwithstanding  their 
immense  velocity. 

Emily,  I  am  surprised  that  so  great  a  velocity  as  20 
degrees  an  hour  should  be  invisible. 

Mrs,  B,  The  real  velocity  depends  altogther  on  the 
space  comprehended  in  each  degree ;  and  this  space  de- 
pends on  the  distance  of  the  object,  and  the  obliquity  of 
its  path.  Observe,  likewise,  that  we  cannot  judge  of  the 
velocity  of  a  body  in  motion  unless  we  know  its  distance  ; 
for  supposing  two  men  to  set  off  at  the  same  moment  from 
A  and  B,  (fig.  2.)  to  walk  each  to  the  end  of  their  respec- 
tive lines  C  and  D  :  if  they  perform  their  walk  in  the 
same  space  of  time,  they  must  have  proceeded  at  a  very 
different  rate,  and  yet  to  an  eye  situated  at  E,  they  will 
appear  to  have  moved  with  equal  velocity  :  because  they 
will  both  have  gone  through  an  equal  number  of  degrees, 
though  over  a  very  unequal  length  of  ground.  Sight  is  an 
extremely  useful  sense  no  doubt,  but  it  cannot  always  be 
relied  on,  it  deceives  us  both  in  regard  to  the  size  and 
the  distance  of  objects  ;  indeed  our  senses  would  be  very 
liable  to  lead  us  into  errour,  if  experience  did  not  set  us 
right. 

Emily,  Between  the  two,  I  think  that  we  contrive  to 
acquire  a  tolerably  accurate  idea  of  objects. 

Mrs,  B,  At  least  sufficiently  so  for  the  general  pur- 
poses of  life.     To  convince  you  how  requisite  experience 

822.     What  must  be  the  velocity  that  its  motion  be  perceptible  ? 

323     Why  is  the  motion  of  the  celestial  bodies  imperceptible  ? 

— r-^624.     What  is  necessary  for  us  to  know  in  order  to  judge  of 

the  A  elocity  of  a  moving  body  ? 825.     In  what  respects  may  the 

sense  of  sight  deceive  us  ? .826.    By  what  are  the  errours  into 

which  we  may  bo  led  by  the  senses  to  be  corrected  ? 


202  ON  THE  ANGLE  OF  VISION. 

is  to  correct  the  errours  of  sight,  I  shall  relate  to  you  the 
case  of  a  young  man  who  was  blind  from  his  infancy,  and 
who  recovered  his  sight  at  the  age  of  fourteen,  by  the  ope- 
ration of  couching.  At  first  he  had  no  idea  either  of  the 
size  or  distance  of  objects,  but  imagined  that  every  thing 
he  saw  touched  his  eyes ;  and  it  was  not  till  after  having 
repeatedly  felt  them,  and  walked  from  one  object  to  ano- 
ther that  he  acquired  an  idea  of  their  respective  dimen-* 
sions,  their  relative  situations,  and  their  distances. 

Caroline,  The  idea  that  objects  touched  his  eyes,  is 
however  not  so  absurd  as  it  at  first  appears  ;  for  if  we 
consider  that  we  see  only  the  image  of  objects,  this  image 
actually  touches  our  e}'es. 

Mrs.  B,  That  is  doubtless  the  reason  of  the  opinion 
he  formed,  before  the  sense  of  touch  had  corrected  his 
judgment. 

Caroline,  But  since  an  image  must  be  formed  on  the 
retina  of  each  of  our  eyes,  why  do  we  not  see  objects 
double  ? 

Mrs,  B,  The  action  of  the  rays  on  the  optick  nerve 
of  each  eye  is  so  perfectly  similar,  that  they  produce  but 
a  single  sensation ;  the  mind  therefore  receives  the  same 
idea,  from  the  retina  of  both  eyes,  and  conceives  the  ob- 
ject to  be  single. 

Caroline.  This  is  difficult  to  comprehend,  and,  I 
should  think,  can  be  but  conjectural. 

3Irs,  B,  I  can  easily  convince  you  that  you  have  a 
distinct  image  of  an  object  formed  on  the  retina  of  each 
eye.  Look  at  the  bell-rope,  and  tell  me,  do  you  see  it  to 
the  right  or  the  left  of  the  pole  of  the  fire-skreen  1 

Caroline.     A  little  to  the  right  of  it. 

Mrs.  B.  Then  shut  your  right  eye,  and  you  will  set 
it  to  the  left  of  the  pole. 

Caroline,     That  is  true  indeed  ! 

Mrs,  B.  There  are  evidently  two  representations  of 
the  bell-rope  in  different  situations,  which  must  be  owing 
to  an  image  of  it  being  formed  on  both  eyes  ;  if  the  action 
of  the  rays  therefore  on  each  retina  were  not  so  perfectly 
similar  as  to  produce  but  one  sensation,  we  should  see 

827.     How  would  objects  appear  as  to  distance,  to  one  who  had 

always  been  blind,  on  first   being  made  to  see  r 828.     Why 

would  they  seem  to  touch  the  eye  ? 829.     If  the  image  of  an 

object  is  formed  on  the  retina  of  each  eye,  why  does  not  the  object 
double ' 


REFLECTING  MIRRORS.  203 

double,  and  we  find  that  to  be  the  case  with  many  persons 
who  are  afflicted  with  a  disease  in  one  eye,  which  pre- 
vents the  rays  of  light  from  affecting  it  in  the  same  man- 
ner as  the  other. 

Emily,  Pray,  Mrs.  B.,  when  we  see  the  image  of  an 
object  in  a  looking-glass,  why  is  it  not  inverted  as  in  the 
camera  obscura,  and  on  the  retina  of  the  eye  ? 

Mrs,  B,  Because  the  rays  do  not  enter  the  mirror  by 
a  small  aperture,  and  cross  each  other,  as  they  do  at  the 
orifice  of  a  camera  obscura,  or  the  pupil  of  the  eye. 

When  yoif  view  yourself  in  a  mirror,  the  rays  from 
your  eyes  fall  perpendicularly  upon  it,  and  are  reflected 
in  the  same  line  ;  the  image  is  therefore  described  behind 
the  glass,  and  is  situated  in  the  same  manner  as  the  ob- 
ject before  it. 

Emily,  Yes,  I  see  that  it  is ;  but  the  looking-glass  is 
not  nearly  so  tall  as  I  am  ;  how  is  it  therefore  that  I  can 
see  the  whole  of  my  figure  in  it  ? 

Mrs,  B,  It  is  not  necessary  that  the  mirror  should  be 
more  than  half  your  height,  in  order  that  you  may  see  the 
whole  of  your  person  in  it  (fig.  3.)  The  ray  of  light  C  D 
from  your  eye,  which  falls  perpendicularly  on  the  mirror 
B  D,  will  be  reflected  back  in  the  same  line  ;  but  the  ray 
from  your  feet  will  fall  obliquely  on  the  mirror,  for  it 
must  ascend  in  order  to  reach  it  ;  it  will  therefore  be  re- 
flected in  the  line  D  A  :  and  since  we  view  objects  in  the 
direction  of  the  reflected  rays,  which  reach  the  eye,  and 
that  the  image  appears  at  the  same  distance  behind  the 
mirror  that  the  object  is  before  it,  we  must  continue  the 
line  A  D  to  E,  and  the  line  C  D  to  F,  at  the  termination 
of  which,  the  image  will  be  represented. 

Emily,  Then  I  do  not  understand  why  I  should  not 
see  the  whole  of  my  person  in  a  much  smaller  mirror,  for 
a  ray  of  light  from  my  feet  would  always  reach  it,  though 
more  obliquely. 

Mrs,  B,  True  ;  but  the  more  obliquely  the  ray  falls 
on  the  mirror,  the  more  obliquely  it  will  be  reflected  ; 


830.     AVhen  we  see  the  image  of  an  object  in  a  looking-glass^ 

why  does  it  not  appear  inverted,  as  in  the  camera  obscura  ? 

831.     What  must  be  the  heii^htof  a  looking  glass,  in  order  for  one 

to  see  his  whole  person  in  it  ? 832.     How  would  you  explain 

Fig"  3,  of  plate  XVII.  ? 833.     Why  may  we  not  see  ourselves 

entire,  in  a  looking-glass  less  than  half  our  height? 


^04  RELFtCTING  MIRRORS. 

the  ray  would  therefore  he  reflected  above  your  head,  and 
you  could  not  see  it.  This  is  shown  by  the  dotted  hne. 
(fig.  3.) 

Now  stand  a  little  to  the  right  of  the  mirror,  so  that 
the  rays  of  light  from  your  figure  may  fall  obliquely  on 
it — 

Emily,  There  is  no  image  formed  of  me  in  the  glass 
now. 

Mrs,  B,  I  beg  your  pardon,  there  is  ;  but  you  cannot 
see  it,  because  the  incident  rays  falling  obliquely  on  the 
mirror  will  be  reflected  obliquely  in  the  opposite  direc- 
tion, the  angles  of  incidence  and  of  reflection  being  equal. 
Caroline,  place  yourself  in  the  direction  of  the  reflected 
rays,  and  tell  me  whether  you  do  not  see  Emily's  image 
in  the  glass  ? 

Caroline,  Let  me  consider.  In  order  to  look  in  the 
direction  of  the  reflected  rays,  I  must  place  myself  as 
much  to  the  left  of  the  glass  as  Emily  stands  to  the  right 
of  it.  Now  I  see  her  image,  but  it  is  not  straight  before 
me,  but  before  her  ;  and  appears  at  the  same  distance 
behind  the  glass,  as  she  is  in  front  of  it. 

Mrs,  B,  You  must  recollect,  that  we  always  see  ob- 
jects in  the  direction  of  the  last  rays  which  reach  our  eyes. 
Figure  4  represents  an  eye  looking  at  the  image  of  a  vase 
reflected  by  a  mirror  ;  it  must  see  it  in  the  direction  of 
the  ray  A  B,  as  that  is  the  ray  which  brings  the  image  to 
the  eye  :  prolong  the  ray  to  C,  and  in  that  spot  will  the 
image  appear. 

Caroline,  I  do  not  understand  why  a  looking-glass  re- 
flects the  rays  of  light :  for  glass  is  a  transparent  body  which 
should  transmit  them. 

Mrs,  B,  It  is  not  the  glass  that  reflects  the  rays  which 
form  the  image  you  behold,  but  the  mercury  behind  it. 
The  glass  acts  chiefly  as  a  transparent  case,  through  which 
the  rays  find  an  easy  passage. 

834.     How  is  this  shown  by  the  figure  ? 835.     Why  cannot 

a  person  see  his  own  image  in  a  looking-glass,  if  he  stand  to  the 

right  or  left  of  it  ? 836.     If  you  stand  obliquely  to  the  right  of 

the  glass,  why  must  another  person  stand  just  as  much  to  the  left 

ol  it,  in  order  to  see  your  image  } 8^37.     When  you  stand  at 

the  right  of  the  glass,  and  I  stand  at  the  left  of  it,  why  does  your 

image  appear  directly  opposite  to  yourself? 83S.     How  would 

you  illustrate  this  by  the  Fiirure  ^- 839.  If  glass  is  a  transpa- 
rent body,  why  will  looking-glasses  reflect  light  ? 


REFLECTION  OF  MIRRORS.  205 

{yaroUne.  Why  then  should  not  mirrors  be  made  sim- 
ply of  mercury  1 

Mrs.  B,  Because  mercury  is  a  fluid.  By  amalgamat- 
ing it  with  tin-foil,  it  becomes  of  the  consistence  of  paste, 
attaches  itself  to  the  glass,  and  forms  in  fact  a  mercurial 
mirror,  which  would  be  much  more  perfect  without  its 
glass  cover  ;  for  the  purest  glass  is  never  perfectly  transpa- 
rent ;  some  of  the  rays  therefore  are  lost  during  their  pas- 
sage through  it,  by  being  either  absorbed,  or  irregularly 
reflected. 

This  imperfection  of  glass  mirrors  has  introduced  the 
use  of  metallick  mirrors,  for  optical  purposes. 

Emily,  But  since  all  opaque  bodies  reflect  the  rays 
of  light,  I  do  not  understand  why  they  are  not  all  mir- 
rors. 

Caroline.  A  curious  idea  indeed,  sister  ;  it  would  be 
very  gratifying  to  see  one's  self  in  every  object  at  which 
one  looked. 

Mrs.  B.  It  is  very  true  that  all  opaque  objects  reflect 
light ;  but  the  surface  of  bodies  in  general  is  so  rough 
and  uneven,  that  their  reflection  is  extremely  irregular, 
which  prevents  the  rays  from  forming  an  image  on  the 
retina.  This  you  will  be  able  to  understand  better,  when 
I  shall  explain  to  you  the  nature  of  vision,  and  the  struc- 
ture of  the  eye. 

You  may  easily  conceive  the  variety  of  directions  in 
which  rays  would  be  reflected  by  a  nutmeg  grater,  on  ac- 
count of  the  inequality  of  its  surface,  and  the  number  of 
holes  with  which  it  is  pierced.  All  solid  bodies  resemble 
the  nutmeg-grater  in  these  respects,  more  or  less  ;  and  it 
is  only  those  which  are  susceptible  of  receiving  a  polish, 
that  can  be  made  to  reflect  the  rays  with  regularity.  As 
hard  bodies  are  of  the  closest  texture,  the  least  porous, 
and  capable  of  taking  the  highest  polish,  they  n-ake  the 
best  mirrors ;  none  therefore  are  so  well  calculated  for 
this  purpose  as  metals. 

Caroline.     But  the  property  of  regular  reflection  is  not 


840.     If  the  mercury  reflect  the  light,  why  should  not  mirrors 

be  made  of  that   material  ? 841.     What  description  of  mirrors 

more  perfect  than  glass  have  been  introduced  ^ 842.     If  all 

opaque  bodies  reflect  light,  why  cannot  we  see  ourselves  as  well 
when  lookincr  at  any  other  object,  as  when  viewing  a  mirror  ? 

r843.     What  substances  make  the  most  perfect  mirrors  ? 

IS 


206        REFLECTION  OP  CONVEX  MIRRORS. 

confined  to  this  class  of  bodies  ;  for  I  have  often  seen  my- 
self in  a  highly  polished  niahogany  table. 

Mrs.  J5.  Certainly  ;  but  as  that  substance  is  less  du- 
rable, and  its  reflection  less  perfect,  than  that  of  metals, 
I  believe  it  would  seldom  be  chosen  for  the  purpose  of  a 
mirror. 

There  are  three  kinds  of  mirrors  used  in  opticks ;  the 
plain  or  flat,  which  are  the  common  mirrors  we  have  just 
mentioned  ;  convex  mirrors ;  and  concave  mirrors. 
The  reflection  of  the  two  latter  is  very  different  from  that 
of  the  former.  The  plain  mirror,  we  have  seen,  does  not 
alter  the  direction  of  the  reflected  rays,  and  forms  an 
image  behind  the  glass  exactly  similar  to  the  object  be- 
fore it.  A  convex  mirror  has  the  peculiar  property  of 
making  the  reflected  rays  diverge,  by  which  means  it  di- 
minishes the  image  ;  and  a  concave  mirror  makes  the 
rays  converge,  and,  under  certain  circumstances,  magni- 
fies the  image. 

Emily.  We  have  a  convex  mirror  in  the  drawing- 
room,  vvbich  forms  a  beautiful  miniature  picture  of  the  ob- 
jects in  the  room  ;  and  T  have  often  amused  myself  with 
looking  at  my  magnif.od  face  in  a  concave  mirror.  But 
I  hope  you  will  explain  to  us  why  the  one  enlarges,  while 
the  other  diminishes  the  objects  it  reflects. 

Mrs.  B.  Let  us  begin  by  examining  the  reflection  of 
a  convex  mirror.  This  is  formed  of  a  portion  of  the  ex- 
teriour  surface  of  a  sphere.  When  several  parallel  rays 
fall  upon  it,  that  ray  only,  which,  if  prolonged,  would 
pass  through  the  centre  or  axis  of  the  mirror,  is  perpen- 
dicular to  it.  In  order  to  avoid  confusion,  I  have  in  fig. 
1,  plate  XVIII.  drawn  only  thre^  parallel  lines,  A  B, 
CD,  E  F,  to  represent  rays  falling  on  the  convex  mirror 
M  N  ;  the  middle  ray,  you  will  obcerve,  is  perpendicular 
to  the  mirror,  the  others  fall  on  it  obliquely. 

Caroline.  As  the  three  rays  are  parallel,  why  are  they 
not  all  perpendicular  to  the  mirror  ? 

Mrs.  B.     They  would  be  so  to  a  flat  mirror  ;  but  as 

844.     How  many  kinds  of  mirrors  are  there  used  in  opticks  ? 

845.     What  are  they  ? 846.     How  does  a  plain  mirror 

exhibit  an   object  ? 847.     How  does  a  convex  mirror  exhibit. 

an  object .'' — --848.  How  does  a  concave  mirror  exhibit  an  ob- 
ject .'■ 849.      Of  what  is  the  convex  mirror  formed  .'' 850. 

What  does  Fig.  1,  plate  XVHI.  represent.' 851.  When  seve- 
ral rays  fall  upon  a  convex  mirror,  which  one  will  be  perpendicu- 
lar to  it  ? 


REFLECTION  OF  CONVEX  MIRRORS,  207 

this  is  spherical,  no  ray  can  fall  perpendicularly  upon  it 
which  is  not  directed  towards  the  centre  of  the  sphere. 

Emily,  Just  as  a  weight  falls  perpendicularly  to  the 
earth  when  gravity  attracts  it  towards  the  centre. 

Mrs.  B,  In  order,  therefore,  that  rays  may  fall  per- 
pendicularly to  the  mirror  at  B  and  F,  the  rays  must  be 
in  the  direction  of  the  dotted  lines,  which,  you  may  ob- 
serve, meet  at  the  centre  O  of  the  sphere,  of  which  the 
mirror  forms  a  portion. 

Now  can  you  tell  me  in  what  direction  the  three  rays. 
A  B,  C  D,  E  F,  will  be  reflected  ? 

Emily.  Yes,  I  think  so  :  the  middle  ray  falling  per- 
pendicularly on  the  mirror,  will  be  reflected  in  the  same 
line  :  the  two  others  falling  obliquely  will  be  reflected 
obliquely  to  G  H  ;  for  the  dotted  lines  you  have  drawn  are 
perpendiculars,  which  divide  their  angles  of  incidence 
and  reflection. 

Mrs,  B.  Extremely  well,  Emily  ;  and  since  we  see 
objects  in  the  direction  of  the  reflected  ray,  we  shall  see 
the  image  at  L,  which  is  the  point  at  which  the  reflected 
rays,  if  continued  through  the  mirror,  would  unite  and 
form  an  image.  This  point  is  equally  distant  from  the 
surface  and  centre  of  the  sphere,  and  is  called  the  imagi- 
nary focus  of  the  mirror. 

Caroline.     Pray  what  is  the  meaning  of  a  focus  l 

Mrs.  B.  A  point  at  which  converging  rays  unite. 
And  it  is  in  this  case  called  an  imaginary  focus ;  be- 
cause the  rays  do  not  really  unite  at  that  point,  but  only 
appear  to  do  so :  for  the  rays  do  not  pass  through  the  mir- 
ror, since  they  are  reflected  by  it. 

Emily.  I  do  not  yet  understand  why  an  object  ap- 
pears smaller  when  viewed  in  a  convex  mirror. 

Mrs.  B.  It  is  owing  to  the  divergence  of  the  reflected 
rays.  You  have  seen  that  a  convex  mirror  converts,  by 
reflection,  parallel  rays  into  divergent  rays ;  rays  that^ 
fall  upon  the  mirror  divergent,  are  rendered  still  more  so 

852.    In  what  direction  must  rays  fall  on  the  convex  mirror  M, 

N,  at  the  points  B,  T,  so  as  to  be  perpendicular  to  it  ? 853. 

Why  will  the  rays  A,  E,  in  Fig.  1,  plate  XVIII.  be  reflected  to  the 

points  G,  H  ? 854.    Why  would  the  image  formed  from  these 

rays  be  seen  at  the  point  L  ? 855.     What  is  the  relative  situa-^ 

tion  of  the  point  L,  and  what  is  it  called  ? 856.     What  is  a  fo- 
cus ? 857.     Why  is  the  point  L    called   an  imaginary  focus .'' 

— — S58.     Why  does  an  object  appear  s-maller  when  viewed  in  a 
convex  mirror  ? 


208  REFLECTION  OP  CONCAVE  MIRRORS. 

by  reflection,  and  convergent  rays  are  reflected  either 
parallel,  or  less  convergent.  If  then  an  object  be  placed 
before  any  part  of  a  convex  mirror,  as  the  vase  A  B,  fig. 
2.  for  instance,  the  two  rays  from  its  extremities,  falling 
convergent  on  the  mirror,  will  be  reflected  less  conver- 
gent, and  will  not  come  to  a  focus  till  they  arrive  at  C  ; 
then  an  eye  placed  in  the  direction  of  the  reflected  rays, 
will  see  the  image  formed  in  (or  rather  behind)  the  mirror 
at  a  b, 

Caroline,  But  the  reflected  rays  do  not  appear  to  me 
to  converge  less  than  the  incident  rays.  I  should  have  sup- 
posed that,  on  the  contrary,  they  converged  more,  since 
they  meet  in  a  point. 

Mrs.  B,  They  would  unite  sooner  than  they  actually 
do,  if  they  were  not  less  convergent  than  the  incident  rays  : 
for  observe,  that  if  the  incident  rays,  instead  of  being  re- 
flected by  the  mirror,  continued  their  course  in  their 
original  direction,  they  would  come  to  a  focus  at  D,  which 
is  considerably  nearer  to  the  mirror  than  at  C  ;  the  image 
is  therefore  seen  under  a  smaller  angle  than  the  object ; 
and  the  more  distant  the  latter  is  from  the  mirror,  the  less 
is  the  image  reflected  by  it. 

You  will  now  easily  understand  the  nature  of  the  re- 
flection of  concave  mirrors.  These  are  formed  of  a  por- 
tion of  the  internal  surface  of  a  hollow  sphere,  and  their 
peculiar  property  is  to  converge  the  rays  of  light. 

Can  you  discover,  Caroline,  in  w  hat  direction  the  three 
parallel  rays,  A  B,  C  D,  E  F,  which  fall  on  the  concave 
mirror  M  N,  (f:g.  3.)  are  reflected  ? 

Caroline.  I  believe  I  can.  The  middle  ray  i&  sent 
back  in  the  same  line,  as  it  is  in  the  direction  of  the  axis 
of  the  mirror  ;  and  the  two  others  will  be  reflected 
obliquely,  as  they  fall  obliquely  on  the  mirror.  I  must 
now  draw  two  dotted  lines  perpendicular  to  their  points 
of  incidence,  which  will  divide  their  angles  of  incidence 
and  reflection ;  and  in  order  that  those  angles  may  be 
equal,  the  two  oblique  rays  must  be  reflected  to  L,  where 
they  will  unite  with  the  middle  ray. 


859.     How  would  you  explain  by  the   Figure,  the  manner  in 
which  a  convex  mirror  makes  an  object  appear  smaller  than  it  is  ? 

860.     Of  what  is  a  concave  mirror  formed  ? 861.     How 

would  you  explain  Fig.  3,  plate  XVHI.  as  illustrating  the  manner 
jn  which  parn.Uel  ray(?\vill  be  reflected  ? 


REFLECTION  OF  CONCAVE  MIRRORS.  ^  209 

Mrs,  B,  Very  well  explained.  Thus  you  see  that, 
when  any  number  of  parallel  rays  fall  on  a  concave  mir- 
ror, they  are  all  reflected  to  a  focus ;  for  in  proportion  as 
the  rays  are  more  distant  from  the  axis  of  the  mirror,  they 
fall  more  obliquely  upon  it,  and  are  more  obliquely  reflect- 
ed ;  in  consequence  of  which  they  come  to  a  focus  in  the 
direction  of  the  axis  of  the  mirror,  at  a  point  equally  dis- 
tant from  the  centre  and  the  surface  of  the  sphere,  and 
this  point  is  not  an  imaginary  focus,  as  happens  with  the 
convex  mirror,  but  is  the  true  focus  at  which  the  rays 
unite. 

Emily.  Can  a  mirror  form  more  than  one  focus  by 
reflecting  rays  1 

Mrs.  B.  Yes.  If  rays  fall  convergent  on  a  concave 
mirror,  (fig.  4.)  they  are  sooner  brought  to  a  focus,  L,  than 
parallel  rays  ;  their  focus  is  therefore  nearer  to  the  mir- 
ror M  N.  Divergent  rays  are  brought  to  a  more  distant 
focus  than  parallel  rays,  as  in  fig.  5.  where  the  focus  is  at 
L ;  but  the  true  focus  of  mirrors,  either  convex  or  con- 
cave, is  that  of  parallel  rays,  which  is  equally  distant  from 
the  centre,  and  the  surface  of  the  sphere. 

I  shall  now  show  you  the  reflection  of  real  rays  of  light, 
by  a  metallick  concave  mirror.  This  is  one  made  of 
polished  tin,  which  I  expose  to  the  sun,  and  as  it  shines 
bright,  we  shall  be  able  to  collect  the  rays  into  a  very 
brilliant  focus.  I  hold  a  piece  of  paper  where  I  imagine 
the  focus  to  be  situated ;  you  may  see  by  the  vivid  spot 
of  light  on  the  paper,  how  much  the  rays  converge ;  but 
it  is  not  yet  exactly  in  the  focus  ;  as  1  approach  the  paper 
to  that  point,  observe  how  the  brightness  of  the  spot  of  light 
increases,  while  its  size  diminishes. 

Caroline.  That  must  be  occasioned  by  the  rays  be- 
coming closer  together.  I  think  you  hold  the  paper  just 
in  the  focus  now,  the  light  is  so  small  and  dazzling — Oh, 
Mrs.  B.,  the  paper  has  taken  fire  ! 

8()2.     Upon  what  does  the  obliquity  depend  with  which  paraJIeJ 

rays  fall  upon  the  surface  of  a  concave  mirror  P 863.     What  is 

the  focus  of  a  concave  mirror  ? 864.     What  is  the  relative  po- 
sition of  the  focus  to  a  concave  mirror  ? 865.     Is  the  focus  of 

a  concave  mirrof  real,  or  only  imaginary  as  in  the  convex  mirror  .'* 

866.     Will  the  focus  be  in  the  same  place  whether  the  rays 

fall  parallel  or  converginsj  upon  the  mirror  ? 867.     Which  is 

most  distant  from  the  mirror  J 868.     Which  figure  illustrates 

tiiis  '<! 869.     Which  will  form  the  more  distant  focus  from  the 

mirror,  divergent  or  parallel  rays.^ 870.     Which  figures  illu^ 

irate  this  ? 

18* 


210  REFLECTION  OF  CONCAVE  MIRRORS. 

Mrs.  B.  The  rays  of  light  cannot  be  concentrated, 
without,  at  the  same  time,  accumulating  a  proportional 
quantity  of  heat :  hence  concave  mirrors  have  obtained 
the  name  of  burning-mirrors. 

Emily,  I  have  often  heard  of  the  surprising  effects  of 
burning-mirrors,  and  I  am  quite  delighted  to  understand 
their  nature. 

Caroline,  It  cannot  be  the  true  focus  of  the  mirror  at 
which  the  rays  of  the  sun  unite,  for  as  they  proceed  from 
a  point,  they  must  fall  divergent  upon  the  mirror. 

Mrs,  B,  Strictly  speaking,  they  certainly  do.  But 
when  rays  come  from  such  an  immense  distance  as  the 
sun,  their  divergence  is  so  trifling,  as  to  be  impercepti- 
ble ;  and  they  may  be  considered  as  parallel :  their  point  of 
union  is,  therefore,  the  true  focus  of  the  mirror,  and  there 
the  image  of  the  object  is  represented. 

Now  that  I  have  removed  the  mirror  out  of  the  influence 
of  the  sun's  rays,  if  I  place  a  burning  taper  in  the  focus, 
how  will  its  light  be  reflected  ?  (fig.  6.) 

Caroline,     That,  I  confess,  I  cannot  say. 

Mrs,  B,  The  ray  which  falls  in  the  direction  of  the 
axis  of  the  mirror,  is  reflected  back  in  the  same  line  ;  but 
let  us  draw  two  other  rays  from  the  focus,  falling  on  the 
mirror  at  B  and  F  ;  the  dotted  lines  are  perpendicular  to 
those  points,  and  the  two  rays  will  therefore  be  reflected 
to  A  and  E. 

Caroline,  Oh,  now  I  understand  it  clearly.  The  rays 
which  proceed  from  a  light  placed  in  the  focus  of  a  con- 
cave mirror  fall  divergent  upon  it,  and  are  reflected  pa- 
rallel. It  is  exactly  the  reverse  of  the  former  experiment, 
in  which  the  sun's  rays  fell  parallel  on  the  mirror,  and  were 
reflected  to  a  focus. 

Mrs.  B.  Yes  :  when  the  incident  rays  are  parallel, 
the  reflected  rays  converge  to  a  focus  ;  when,  on  the  con- 
trary, the  incident  rays  proceed  from  the  focus,  they  are 
reflected  parallel.  This  is  an  important  law  of  opticks, 
and  since  you  are  now  acquainted  with  the  principles  on 
which  it  is  founded,  I  hope  that  you  will  not  forget  it. 

871.      What  are  concave  mirrors  sometimes  called? 672. 

Why    are   they  called   burning-glasses  ? 873.      Do   the  rays 

which  come  from  the  sun,  on  being  reflected  by  a  concave  mirror, 

meet  in  the  true  focus  of  the  mirror  ? r874.     If  a  burning  taper 

is  placed  in  the  focus  of  a  concave  mirror,  how  will  its  lig^ht  be  re- 
flected ^- 875.     What  is  illustrated  by  Fig.  6,  plate  XVIII. .?-- — 

876.     What  is  mentioned  as  an  important  law  in  opticks  relating 
to  the  falling  of  light  upon  mirrors  ? 


I 


THE  REFRACTION  OF  LIGHT.  211 

Caroline.  I  am  sure  that  we  shall  not.  But,  Mrs.  B., 
you  said  that  the  image  was  formed  in  the  focus  of  a  con- 
cave mirror  ;  yet  I  have  frequently  seen  glass  concave 
mirrors,  where  the  object  has  been  represented  within  the 
mirror,  in  the  same  manner  as  in  a  convex  mirror. 

Mrs.  B.  That  is  the  case  only,  when  the  object  is 
placed  between  the  mirror  and  its  focus ;  the  image  then 
appears  magnified  behind,  or,  as  you  call  it,  within  the 
mirror. 

Caroline.  I  do  not  understand  why  the  image  should 
be  larger  than  the  object. 

Mrs.  B.  It  proceeds  from  the  convergent  property  of 
the  concave  mirror.  If  an  object,  A  B,  (fig.  7.)  be  placed 
between  the  mirror  and  its  focus,  the  rays  from  its  extre- 
mities fall  divergent  on  the  mirror,  and  on  being  reflected, 
become  less  divergent,  as  if  they  proceeded  from  C  :  to 
an  eye  placed  in  that  situation  the  image  will  appear  mag- 
nified behind  the  mirror  at  a  6,  since  it  is  seen  under  a 
larger  angle  than  the  object. 

You  now,  I  hope,  understand  the  reflection  of  light  by 
opaque  bodies.  At  our  next  meeting,  we  shall  enter  upon 
another  property  of  light  no  less  interesting,  which  is  call- 
ed refraction. 


CONVERSATION  XVI. 

ON  REFRACTION  AND  COLOURS. 

Transmission  of  Light  hy  Transparent  Bodies  ;  Refrac- 
tion; Refraction  of  the  Atmosphere  ;  Refraction  of  a 
Lens ;  Refraction  of  the  Prism ;  Of  the  Colours 
of  Rays  of  Light ;   Of  the  Colours  of  Bodies. 

MRS.  B. 

The  refraction  of  light  will  furnish  the  subject  of  to- 
day's lesson. 

Caroline.  This  is  a  property  of  which  I  have  not  the 
faintest  idea. 

877.     Where  must  the   object  be  placed  in  regard  to  a  concave 

toirror,  in  order   that  the  image  appear  behind  the  mirror  ? 

878.     Why  does  the  image  in  a  concave  mirror  appear  larger  than 
the  object  f— '87i>.    How  is  this  illustrated  by  the  figure  ? 


212  THE  REFRACTION  OF  LIGHT. 

Mrs.  B,  It  is  the  effect  which  transparent  mediums 
produce  on  light  in  its  passage  through  them.  Opaque 
bodies,  you  know,  reflect  the  rays,  and  transparent  bodies 
transmit  them  ;  but  it  is  found,  that  if  a  ray,  in  passing 
from  one  medium  into  another  of  different  density,  fall 
obliquely,  it  is  turned  out  of  its  course. 

Caroline.  It  must  then  be  acted  on  by  some  new  power, 
otherwise  it  would  not  deviate  from  its  first  direction. 

Mrs.  B.  The  power  which  causes  the  deviation  of 
the  ray  appears  to  be  the  attraction  of  the  denser  medium. 
Let  us  suppose  the  two  mediums  to  be  air  and  water  ;  if 
a  ray  of  light  passes  from  air  into  water,  it  is  more  strong- 
ly attracted  by  the  latter  on  account  of  its  superiour  den- 
sity. 

Emily.  In  what  direction  does  the  water  attract  the 
ray? 

Mrs.  B.  It  must  attract  it  perpendicularly  towards  it 
in  the  same  manner  as  gravity  acts  on  bodies. 

If  then  a  ray  A  B,  (fig.  1,  plate  XIX.)  fall  perpendicu- 
larly on  water,  the  attraction  of  the  water  acts  in  the  same 
direction  as  the  course  of  the  ray  :  it  will  not  therefore 
cause  a  deviation,  and  the  ray  will  proceed  straight  on  ta 
E.  But  if  it  fall  obliquely,  as  the  ray  C  B,  the  water  will 
attract  it  out  of  its  course.  Let  us  suppose  the  ray  to 
have  approached  the  surface  of  a  denser  medium,  and 
that  it  there  begins  to  be  affected  by  its  attraction  ;  this 
attraction,  if  not  counteracted  by  some  other  power,  would 
draw  it  perpendicularly  to  the  water,  at  B  ;  but  it  is  also 
impelled  by  its  projectile  force,  which  the  attraction  of 
the  denser  medium  cannot  overcome  ;  the  ray,  therefore, 
acted  on  by  both  these  powers,  moves  in  a  direction  be- 
tween them,  and  instead  of  pursuing  its  original  course  to 
D,  or  being  implicitly  guided  by  the  water  to  E.  proceeds 
towards  F,  so  that  the  ray  appears  bent  or  broken. 

Caroline.  I  understand  that  very  well ;  and  is  not  this 
the  reason  that  oars  appear  bent  in  water  1 


880.     What  is  meant  by  the  refraction  of  light  ^ 881.     When 

does  refraction  in  hght  take  place  ? 882.     What  power  causes 

the  refraction  of  light  ? 883.     How  would  you  illustrate  the  re- 
fraction of  light  by  an  explanation  of  Fig.  1,  plate  XIX.  .'' 884 

Why  does  the  ray  C  B  desceBd  to  F  instead  of  D  or  E  in  tlH^^ 
figure  ' 


THE  RErRACTION  OF  LIGHT.  213 

Mrs,  B,  It  is  owing  to  the  refraction  of  the  rays  re- 
flected by  the  oar ;  but  this  is  in  passing  from  a  dense  to 
a  rare  medium,  for  you  know  that  the  rays,  by  means  of 
which  you  see  the  oar,  pass  from  water  into  air. 

E,mU.y.     Bat  I  do  not  understand   why   a  refraction 
takes  place  when  a  ray  passes   from  a  dense  into  a  rare  ^^^ 
medium ;  I  should  suppose  that  it  would  be  rather  less^ 
than  more  attracted  by  the  latter. 

Mrs,  B,  And  it  is  precisely  on  that  account  that  the 
ray  is  refracted.  C  B,  fig.  2,  represents  a  ray  passing  ob* 
liquely  from  glass  into  water  :  glass  being  the  denser  me- 
dium, the  ray  will  be  more  strongly  attracted  by  that  which 
it  leaves  than  by  that  which  it  enters.  The  attraction  of 
the  glass  acts  in  the  direction  A  B,  while  the  impulse  of 
projection  would  carry  the  ray  to  F ;  it  moves  therefore 
between  these  directions  towards  D. 

Emily,  So  that  a  contrary  refraction  takes  place  when 
a  ray  passes  from  a  dense  into  a  rare  medium. 

Caroline.  But  does  not  the  attraction  of  the  denser 
medium  affect  the  ray  before  it  touches  it  ? 

Mrs,  B,  The  distance  at  which  the  attraction  of  the 
denser  medium  acts  upon  a  ray  is  so  small  as  to  be  insen- 
sible ;  it  appears  therefore  to  be  refracted  only  at  the  point 
at  which  it  passes  from  one  medium  to  the  other. 

Now  that  you  understand  the  principle  of  refraction,  I 
will  show  you  the  refraction  of  a  real  ray  of  light.  Do 
you  see  the  flower  painted  at  the  bottom  of  the  inside  of 
this  tea-cup  ?  (Fig.  3.) 

Emily,  Yes.  But  now  you  have  moved  it  just  out  of 
sight  ;  the  rim  of  the  cup  hides  it. 

Mrs,  B,  Do  not  stir.  I  will  fill  the  cup  with  v/ater, 
and  you  will  see  the  flower  again. 

Emily,  I  do  indeed  !  Let  me  try  to  explain  this : 
when  you  drew  the  cup  from  me  so  as  to  conceal  the  flow- 
er, the  rays  reflected  by  it  no  longer  met  my  eyes,  but 
were  directed  above  them  ;  but  now  that  you  have  filled 
the  cup  with  water,  they  are  refracted  by  the  attraction 
of  the  water,  and  bent  downwards  so  as  again  to  enter  my 
eyes. 

Mrs,  B,     You  have   explained  it  perfectly  :    Fig.   3. 

885.     Why  does  a  straight  stick  appear  crooked  when  one  end 

of  it  is  immersed  obliquely  in  the  water  ? 88().      How  would 

you  explain  Fi«T.  2,  plate  XIX.  ^ 887.      Does  the  attraction  of 

the  denser  medium  affect  the  ray  before  it  touches  it  ? 


214  THE  REFRACTION  OF  LIGHT. 

will  help  to  imprint  it  on  your  memory.  You  must  ob- 
serve tiiai  when  the  flower  becomes  visible  by  the  refrac- 
tion of  the  ray,  you  do  not  see  it  in  the  situation  which  it 
really  occupies,  but  an  image  of  the  flower  higher  in  the 
cup ;  for  as  objects  always  appear  to  be  situated  in  the 
iBMirection  of  the  rays  which  enter  the  eye,  the  flower  will 
be  seen  in  the  direction  of  the  reflected  ray  at  B. 

Emily,  Then  when  we  see  the  bottom  of  a  clear 
stream  of  water,  the  rays  which  it  reflects  being  refracted 
in  their  passage  from  the  water  into  the  air,  will  make  the 
bottom  appear  higher  than  it  really  is. 

3Irs,  B.  And  the  water  will  consequently  appear 
more  shallow.  Accidents  have  frequently  been  occasion- 
ed by  this  circumstance ;  and  boys  who  are  in  the  habit 
of  bathing  should  be  cautioned  not  to  trust  to  the  appa- 
rent shallowness  of  water,  as  it  will  always  prove  deeper 
than  it  appears  ;  unless  indeed,  they  view  it  from  a  boat 
on  the  water,  which  will  enable  them  to  look  perpendicu- 
larly upon  it ;  v*'hen  the  rays  from  the  bottom  passing  per- 
pendicularly, no  refraction  will  take  place. 

The  refraction  of  light  prevents  our  seeing  the  heaven- 
ly bodies  in  their  real  situation ;  the  light  they  send  to  us 
being  refracted  in  passing  into  the  atmosphere,  we  see  the 
sun  and  stars  in  the  direction  of  the  refracted  ray  ;  as  de- 
scribed in  fig.  4,  plate  XIX. ;  the  dotted  line  represents 
the  extent  of  the  atmosphere,  above  a  portion  of  the  earth 
E  B  E  :  a  ray  of  light  coming  from  the  sun  S  falls  ob- 
liquely on  it,  at  A,  and  is  refracted  to  B  :  then  since  we 
see  the  object  in  the  direction  of  the  refracted  ray,  a  spec- 
tator at  B  will  see  an  image  of  the  sun  at  C,  instead  of 
the  real  object  as  S. 

Emily.  But  if  the  sun  were  immediately  over  our 
heads,  its  rays  falling  perpendicularly  on  the  atmosphere 
would  not  be  refracted,  and  we  should  then  see  the  real 
sun  in  its  true  situation. 

Mrs,  B,  You  must  recollect  that  the  sun  is  vertical 
only  to  the  inhabitants  of  the  torrid  zone ;  its  rays,  there- 

888.  How  would  you  describe  the  experiment  represented  in 
Fig.  3,  plate  XIX.  ? 889.  Why  does  water  appear  more  shal- 
low than  it  really  is  ? 890.     In  what  situation  may  the  bottom 

of  water  be  viewed  so  as  to  appear  of  its  real  depth  ? 891.     Do 

we  see  the  heavenly  bodies  in  their  real  situation  .•' 892.     Why 

do  we  not  ? 893.     By  which  Figure  is  this  illustrated,  and  how 

would  you  describe  the  illustration  given  ? 894.     In  what  si- 
tuation may  the  sun  be  seen  in  ils  true  place  ? 


THE  REFRACTION  OF  LIGHT.  215 

fore,  are  always  refracted  in  these  climates.  There  is 
also  another  obstacle  to  our  seeing  the  heavenly  bodies  in 
their  real  situations  ;  light,  though  it  moves  with  extreme 
velocity,  is  about  eight  minutes  and  a  half  in  its  passage 
from  the  sun  to  the  earth  ;  therefore,  when  the  rays  reac' 
us,  the  sun  must  have  quitted  the  spot  he  occupied 
their  departure  ;  yet  we  see  him  in  the  direction  of  thosS 
rays,  and  consequently  in  a  situation  which  he  had  aban- 
doned eight  minutes  and  a  half  before. 

Etnily,  When  you  speak  of  the  sun's  motion,  you 
mean,  I  suppose,  his  apparent  motion,  produced  by  the 
diurnal  motion  of  the  earth  1 

Mrs,  B.  No  doubt ;  the  effect  being  the  same,  whether 
it  is  our  earth,  or  the  heavenly  bodies  which  move :  it  ife 
more  easy  to  represent  things  as  they  appear  to  be,  than 
as  they  really  are. 

Caroline,  During  the  morning,  then,  when  the  sun  is 
rising  towards  the  meridian,  we  must  (from  the  length  of 
time  the  light  is  in  reaching  us)  see  an  image  of  the  sun 
below  that  spot  whic]^  it  really  occupies. 

Emily.  But  the  refraction  of  the  atmosphere  counter- 
acting this  effect,  w^e  may  perhaps,  between  the  two,  see 
the  sun  in  its  real   situation. 

Caroline,  And  in  the  afternoon,  when  the  sun  is  sink- 
ing in  the  west,  refraction  and  the  length  of  time  which 
the  light  is  in  reaching  the  earth,  will  conspire  to  render 
the  image  of  the  sun  higher  than  it  really  is. 

Mrs,  B,  The  refraction  of  the  sun's  rays  by  the  at- 
mosphere prolongs  our  days,  as  it  occasions  our  seeing  an 
image  of  the  sun  both  before  he  rises  and  after  he  sets ; 
for  below  the  horizon,  he  still  shines  upon  the  atmosphere, 
and  his  rays  are  thence  refracted  to  the  earth.  So  like- 
wise we  see  an  image  of  the  sun  before  he  rises,  the  rays 
that  previously  fall  upon  the  atmosphere  being  reflected  to 
the  earth.* 


"  It  is  entirely  owing  to  the  reflection  of  the  atmosphere  that 
the  heavens  appear  bright  in  the  day  time.  For  without  it,  oaly 
that  part  would  be  luminous  in  which   the  sun  is  placed  ;  and  if 

805.  How  long  is  light  in  coming  from  the  sun  to  the  earth  ? 
896.  How  would  you  explain  the  effect  this  has  on  the  ap- 
parent situation  of  that  luminary  ? 897.     What  effect  does  the 

refraction  of  light  from  the  atmosphere  have  on  the  length  of  our 

days .'' 898.      What   would  he   the  appearance   of  the  heavens 

were  it  not  for  the  atmosphere  f 


^16  THE  REFRACTION  OF  LIGHT. 

Carolint,  On  the  other  hand  we  must  recollect  thai 
iight  is  eight  minutes  and  a  half  on  its  journey  ;  so  that,  by 
the  time  it  reaches  the  earth,  the  sun  may  perhaps  be  risen 
above  the  horizon. 

Emily.     Pray,  do  not  glass  windows  refract  the  light  ? 

Mrs,  B,  They  do ;  but  this  refraction  is  not  percep- 
tible, because,  in  passing  through  a  pane  of  glass,  the  rays 
suffer  two  refractions,  which  being  in  contrary  directions, 
produce  the  same  effect,  as  if  no  refraction  had  taken 
place. 

Emily,     I  do  not  understand  that. 

Mrs.  B.  Fig.  5,  plate  XIX.  will  make  it  clear  to 
you  :  A  A  represents  a  thick  pane  of  glass  seen  edgeways. 
When  the  ray  B  approaches  the  glass  at  C,  it  is  refracted 
by  it ;  and  instead  of  continuing  its  course  in  the  same  di- 
rection, as  the  dotted  line  describes,  it  passes  through  the 
pane  to  D ;  at  that  point  returning  into  the  air,  it  is  again 
refracted  by  the  glass,  but  in  a  contrary  direction  to  the 
first  refraction,  and  in  consequence  proceeds  to  E.  Now 
you  must  observe  that  the  ray  B  C  and  the  ray  D  E  being 
parallel,  the  light  does  not  appear  to  have  suffered  any 
refraction. 

Emily.  So  that  the  effect  which  takes  place  on  the 
ray  entering  the  glass,  is  undone  on  its  quitting  it.  Or, 
to  express  myself  more  scientifically,  when  a  ray  of  light 
passes  from  one  medium  into  another,  and  through  that 
into  the  first  again,  the  two  refractions  being  equal  and  in 
opposite  directions,  no  sensible  effect  is  produced. 

Mts.  B.  This  is  the  case  when  the  two  surfaces  of 
the  refracting  medium  are  parallel  to  each  other  ;  if  they 
are  not,  the  two  refractions  may  be  made  in  the  same  di- 
rection, as  I  shall  show  you. 


We  could  live  without  air,  and  should  turn  our  backs  to  the  sun,  the 
whole  heavens  would  appear  as  dark  as  in  the  night.  In  this  case, 
also,  we  should  have  no  twilig^ht,  but  a  sudden  transition  from  the 
brightest  sunshine  to  dark,  immediately  upon  the  setting  of  the 
sun. 


899.  In  what  manner  wovld  the  changes  of  day  and  night  then 
take  place  ? 900.  Is  light  refracted  in  passing  throngli  com- 
mon vi^indow-glass  ? 901.     Why  then  is  not  the  refraction  per 

ceptible  ? 902.     Which  figure  illustrates  this  ? 


THE  REFRACTION  OP  LIGlfT*  217 

When  parallel  rays  (fig.  6.)  fall  on  a  piece  of  glass  hav- 
ing a  double  convex  surface,  and  which  is  called  a  Lens^ 
that  only  which  falls  in  the  direclion  of  the  axis  of  the 
lens  is  perpendicular  to  the  surface  ;  the  other  rays  fall* 
ing  obliquely,  are  refracted  towards  the  axis,  and  will 
meet  at  a  point  beyond  the  lens,  called  its  focus.  { 

Of  the  three  rays,  ABC,  which  fall  on  the  lens  D  E, 
the  rays  A  and  C  are  refracted  in  their  passage  through 
it,  to  a  and  c,  and  on  quitting  the  lens  they  undergo  a  se- 
cond refraction  in  the  same  direction  which  unites  them 
with  the  ray  B  at  the  focus  F. 

Emihj,  And  what  is  the  distance  of  the  focus  from 
the  surface  of  the  lens  ? 

Mrs,  B,  The  focal  distance  depends  both  upon  the 
form  of  the  lens,  and  of  the  refractive  power  of  the  sub- 
stance of  which  it  is  made  ;  in  a  glass  lens,  both  sides  of 
which  are  equally  convex,  the  focus  is  situated  nearly  at 
the  centre  of  the  sphere  of  which  the  surface  of  the  lens 
forms  a  portion  ;  it  is  at  the  distance,  therefore,  of  the  ra- 
dius of  the  sphere. 

There  are  lenses  of  various  forms,  as  you  will  find  de- 
scribed in  fig.  1,  plate  XX.  The  property  of  those  which 
have  a  convex  surface  is  to  collect  the  rays  of  light  to  a 
focus  ;  and  of  those  which  have  a  concave  surface,  on  the 
contrary,  to  disperse  thenu  For  the  rays  A  C  falling  on 
the  concave  lens  X  Y,  (fig.  7,  plate  XIX. ,)  instead  of  con- 
verging towards  the  ray  B,  which  falls  on  the  axis  of  the 
lens,  will  each  be  attracted  towards  the  thick  edges  of  the 
lens,  both  on  entering  and  quitting  it,  and  will,  therefore, 
by  the  first  refraction,  be  made  to  diverge  to  a,  c,  and  by 
the  second  to  d,  e, 

Caroline,  And  lenses  which  have  one  side  flat  and  the 
other  convex  or  concave,  as  A  and  B,  fig.  1,  plate  XX. 
are,  I  suppose,  less  powerful  in  their  refractions. 

Mrs,  B.  Yes ;  they  are  called  plano-convex,  and 
plano-concave  lenses ;  the  focus  of  the  former  is  at  the 

903.    What  is  a  lens  ? 904    In  parallel  rays  that  pass  through 

a  lens  what  ones  will  be  refracted  ? 905.     In  what   place  \yill 

the  refracted  rays   meet  ? 906.     Which  figure  illustrates  this  ^ 

— — 907.     What  is  the  distance  of  the  focus  from  the  surface  of 

the  lens  ? 908.     What  is  the  property  of  a  convex  lens  ? 

909.     What   is  the  property  of  a  concave  lens  = 910.     What 

does  Figure  7,  Plate  six  illustrate  .? 911.     What  is  a  plano-con- 
vex lens .' 

19 


218  ON  REFRACTION  AND  COLOURS. 

distance  of  the  diameter  of  a  sphere,  of  which  the  convex 
surface  of  the  lens  forms  a  portion  ;  as  represented  in 
fig.  2,  plate  XX.  The  three  parallel  rays,  ABC,  are 
brought  to  a  focus  by  the  plano-convex  lens,  X  Y  at  F. 
I  must  now  explain  to  you  the  refraction  of  a  triangular 
'piece  of  glass,  called  a  prism.     (Fig.  3.) 

Eniihj,  The  three  sides  of  this  glass  are  flat;  it  can- 
not therefore  bring  the  rays  to  a  focus  ;  nor  do  I  suppose 
that  its  refraction  will  be  similar  to  that  of  a  flat  pane  of 
glass,  because  it  has  not  two  sides  parallel ;  I  cannot 
therefore  conjecture  what  effect  the  refraction  of  a  prism 
can  produce. 

Mrs,  B,  The  refractions  of  the  light,  on  entering  and 
on  quitting  the  prism,  are  both  in  the  same  direction. 
(Fig.  3.)  On  entering  the  prism  P,  the  ray  A  is  refracted 
from  B  to  C,  and  on  quitting  it  from  C  to  D. 

I  will  show  you  this  in  nature ;  but  for  this  purpose  it 
will  be  adviseable  to  close  the  window-shutters,  and  ad- 
mit, through  the  small  aperture,  a  ray  of  light,  which  I 
shall  refract  by  means  of  this  prism. 

Caroline,  Oh,  what  beautiful  colours  are  represented 
on  the  opposite  w'all  !  There  are  all  the  colours  of  the  rain- 
bow, and  with  a  brightness  I  never  saw  equalled.  (Fig. 
4,  plate  XX.) 

Emily,  I  have  seen  an  effect,  in  some  respect  similar 
to  this,  produced  by  the  rays  of  the  sun  shining  upon  glass 
lustres ;  but  how  is  it  possible  that  a  piece  of  white  glass 
can  produce  such  a  variety  of  brilliant  colours  ? 

Mrs,  B,  The  colours  are  not  formed  by  the  prism,  but 
existed  in  the  ray  previous  to  its  refraction. 

Caroline,  Yet,  before  its  refraction,  it  appeared  per- 
fectly white. 

Mrs,  B.  The  white  rays  of  the  sun  are  composed  of 
coloured  rays,  which,  when  blended  together,  appear  co- 
lourless or  white. 

Sir  Isaac  Newton,  to  whom  w^e  are  indebted  for  the 
most  important  discoveries  respecting  light  and  colours, 

912.     What  is  a  plano-concave  lens? 913.     Where  will  be 

the  focus  of  a  plano-convex  lens  ' 914.     What  is  illustrated  by- 
figure  2,  plate  XX.  => 915.     What  is  a  prism .^ 916.     What 

does  figure  3,  plate  XX.  represent  ^ 917.      What  is  the  design 

of  figure  4,  plate  XX. : 918.  Are  the  different  colours  exhibited 

in  that   figure  formed  by   the   prism  .^ 919.     Of  what  are  the 

white  rays  of  the  sun  composed  ? 920.    To  whom  arc  we  inuebt- 

ed  for  the  most  important  discoveries  respecting  light  and  colours .' 


ON  REFRACTION  AND  COLOURS.  219 

was  the  first  who  divided  a  white  ray  of  light,  and  found 
it  to  consist  of  an  assemblage  of  coloured  rays,  which 
formed  an  image  upon  the  wall,  such  as  you  now  see  ex- 
hibited, (fig.  4.)  in  which  are  displayed  the  following  se- 
ries of  colours :  red,  orange,  yellow,  green,  blue,  indigo^  . 
and  violet.  ^'0 

Emily,  But  how  does  a  prism  separate  these  coloured 
rays  ? 

Mrs.  B.  By  refraction.  It  appears  that  the  coloured 
rays  have  different  degrees  of  refrangibility  ;  in  passing 
through  the  prism,  therefore,  they  take  different  direc- 
tions according  to  their  susceptibility  of  refraction.  The 
violet  rays  deviate  most  from  their  original  course  ;  they 
appear  at  one  of  the  ends  of  the  spectrum  A  B  :  contigu- 
ous to  the  violet,  are  the  blue  rays,  being  those  which 
have  somewhat  less  refrangibility  :  then  follow,  in  succes- 
sion, the  green,  yellow,  orange,  and,  lastly,  the  red,  which 
are  the  least  refrangible  of  the  coloured  rays. 

Caroline,  I  cannot  conceive  how  these  colours,  mixed 
together,  can  become  white. 

Mn,  B,  That  I  cannot  pretend  to  explain  ;  but  it  is 
a  fact  that  the  union  of  these  colours,  in  the  proportions  in 
which  they  appear  in  the  spectrum,  produce  in  us  the 
idea  of  whiteness.  If  you  paint  a  card  in  compartments 
with  these  seven  colours,  and  whirl  it  rapidly  on  a  pin,  it 
will  appear  white. 

But  a  more  decisive  proof  of  the  composition  of  a  white 
ray  is  afforded  by  re-uniting  these  coloured  rays,  and  form- 
ing with  them  a  ray  of  white  light.* 


*  The  same  conclusion  may  be  drawn  from  the  experiment  of 
mixing  together  paints  of  the  colours  exhibited  in  the  prism,  and 
in  proper  proportions,  which  will  form  white.  It  is  true  the  white 
will  not  be  of  the  resplendent  kind  ;  but  this  will  be  owing  to  the 
colours  mixed  being  less  bright  than  those  produced  from  the 
prism. 

921.     What  is  the  order  of  the  colours  displayed  in  the  prism  ? 

922.     How  does  the  prism  separate  these  rays  ? 923.     To 

what  is  the  different  directions,  taken  by  the  different  rays  in  pass- 
ing through  a  prism,  owing  ? 924.     Which  rays  deviate  most 

and  which  least  from  their  original  course  in  passing  through  a 

prism? 925.     What  fact  is  mentioned  respecting  a  painted 

card,  as  proving  that  these  seven  colours  united  make  white  .'' 

926.     What  experiment  relating  to  this  subject  is  mentioned  in  the 
note  f 


220  ON  REFRACTION  AND  COLOURS. 

Caroline.  If  you  can  take  a  ray  of  white  light  to  pieces, 
and  put  it  together  again,  I  shall  be  quite  satisfied. 

Mrs.  B.  This  can  be  done  by  letting  the  coloured 
rays,  which  have  been  separated  by  a  prism,  fall  upon  a 
lens,  which  will  converge  them  to  a  focus  ;  and  if,  when 
w^  thus  re-united,  we  find  that  they  appear  white  as  they  did 
before  refraction,  I  hope  that  you  will  be  convinced  that 
the  white  rays  are  a  compound  of  the  several  coloured  rays. 
The  prism  P,  you  see,  (fig.  5.)  separates  a  ray  of  white 
light  into  seven  coloured  rays,  and  the  lens  L  L  brings 
them  to  a  focus  at  F,  where  they  again  appear  white. 

Caroline.  You  succeed  to  perfection  :  this  is  indeed  a 
most  interesting  and  conclusive  experiment. 

Emily.  Yet,  Mrs.  B.,  I  cannot  help  thinking,  that 
there  may  perhaps  be  but  three  distinct  colours  in  the 
spectrum,  red,  yellow,  and  blue  ;  and  that  the  four  others 
may  consist  of  two  of  these  colours  blended  together ;  for 
in  painting,  we  find  that  by  mixing  red  and  yellow,  we  pro- 
duce orange  ;  with  different  proportions  of  red  and  blue, 
we  make  violet  or  any  shade  of  purple  ;  and  yellow  and 
blue  form  green.  Now  it  is  very  natural  to  suppose,  that 
the  refraction  of  a  prism  may  not  be  so  perfect  as  to  se- 
parate the  coloured  rays  of  light  completely,  and  that  those 
which  are  contiguous  in  order  of  refrangibility  may  en- 
croach on  each  other,  and  by  mixing  produce  the  inter- 
mediate colours,  orange,  green,  violet,  and  indigo. 

Mrs.  B.  Your  observation  is,  I  believe,  neither  quite 
wrong,  nor  quite  right.  Dr.  Wollaston,  who  has  refract- 
ed light  in  a  more  accurate  manner  than  had  been  pre- 
viously done,  by  receiving  a  very  narrow  line  of  light  on  a 
prism,  found  that  it  formed  a  spectrum,  consisting  of  rays 
of  four  colours  only  ;  but  they  were  not  exactly  those  you 
have  named  as  primitive  colours,  for  they  consisted  of  red, 
green,  blue,  and  violet.  A  very  narrow  line  of  yellow 
was  visible,  at  the  limit  of  the  red  and  green,  which  Dr. 
Wollaston  attributed  to  the  overlapping  of  the  edges  of 
the  red  and  green  light. 


927.     How  can  these  colours  once  separated  be  again  united  ? 

928.     Which  figure  illustrates  this  ? 929.     Who  has  been 

very  successful  and  accurate  in  experiments  upon    the  refrac- 
tion of  light  ? 930.    Wlmt  did  he  suppose  to  be  the  primitive 

colours  ? 


I 


ON  REFRACTION  AND  COLOURS.  231 

Caroline,  But  red  and  green,  mixed  together,  do  not 
produce  yellow. 

Mrs,  B.  Not  in  painting  ;  but  it  may  be  so  in  the 
primitive  rays  of  the  spectrum.  Dr.  WoUaston  observed 
that,  by  increasing  the  breadth  of  the  aperture  by  which 
the  line  of  light  was  admitted,  the  space  occupied  by  each 
coloured  ray  in  the  spectrum  was  augmented  in  proportion 
as  each  portion  encroached  on  the  neighbouring  colour 
and  mixed  with  it ;  so  that  the  intervention  of  orange  and 
yellow,  between  the  red  and  green,  is  owing,  he  supposes, 
to  the  mixture  of  these  two  colours,  and  the  blue  is  blend- 
ed on  the  one  side  with  the  green,  and  on  the  other 
with  the  violet,  forming  the  spectrum  as  it  was  originally 
observed  by  Sir  Isaac  Newton,  and  which  I  have  just 
shown  you. 

The  rainbow,  which  exhibits  a  series  of  colours  so  ana- 
logous to  those  of  the  spectrum,  is  formed  by  the  refraction 
of  the  sun's  rays  in  their  passage  through  a  shower  of 
rain,  every  drop  of  which  acts  as  a  prism,  in  separating 
the  coloured  rays  as  they  pass  through  it.* 

Emily,  Pray,  Mrs.  B.,  cannot  the  sun's  rays  be  col- 
lected to  a  focus  by  a  lens  in  the  same  manner  as  they  are 
by  a  concave  mirror  ? 

Mrs,  B,  No  doubt  the  same  effect  is  produced  by  the 
refraction  of  a  lens  as  by  the  reflection  of  a  concave  mir- 
ror :  in  the  first,  the  rays  pass  through  the  glass  and  con- 
verge to  a  focus  behind  it ;  in  the  latter,  they  are  reflect- 
ed from  the  mirror,  and  brought  to  a  focus  before  it.  A 
lens,  when  used  for  the  purpose  of  collecting  the  sun's 
rays,  is  called  a  burning  glass.  The  sun  now  shines  very 
bright;  if  we  let  the  rays  fall  on  this  lens  you  will  per- 
ceive the  focus. 


*  That  this  is  the  true  account  of  the  formation  of  the  rainbow 
appears  from  the  following  considerations — 1.  That  a  bow  is  never 
seen  "but  when  rain  is  falling,  and  the  sun  shining  at  the  same 
time,  and  that  the  sun  and  bow  are  always  in  opposite  parts  of 
the  heavens  ;  and,  secondly,  that  the  same  appearance  can  be 
artificially  represented  by  means  of  water  thrown  into  the  air,  when 
the  spectator  is  placed  in  a  proper  position  with  his  back  towards 
the  sun. 


931.     How  is  the  rain-bow  formed  ? 932.     From  what  COU' 

sidcrations  does  it  appear  that  the  rain-bow  is  formed  by  the  re- 
fraction  of  the  sun's  rays  in  their  passage  through  a  shower  of 

rain  ? ^933     When  is  a  lens  called  a  burning  glass  ? 

19* 


222  ON  REFRACTION  AND  COLOURS. 

Emily.  Oh  yes  ;  the  point  of  union  of  the  rays  is  very 
luminous.  I  will  hold  a  piece  of  paper  in  the  focus,  and 
see  if  it  will  take  fire.  The  spot  of  light  is  extremely 
brilliant,  but  the  paper  does  not  burn. 

Mrs,  B.  Try  a  piece  of  brown  paper  ; — that  you  see 
takes  fire  almost  immediately. 

Caroline,  This  is  surprising  ;  for  the  light  appeared 
to  shine  more  intensely  on  the  white  than  on  the  brown 
paper. 

Mrs.  B.  The  lens  collects  an  equal  number  of  rays  to 
a  focus,  whether  you  hold  the  white  or  the  brown  paper 
there  ;  but  the  white  paper  appears  more  luminous  in  the 
focus,  because  most  of  the  rays,  instead  of  entering  into 
the  paper,  are  reflected  by  it ;  and  this  is  the  reason  that 
the  paper  is  not  burnt ;  whilst  on  the  contrary,  the  brown 
paper,  which  absorbs  more  light  than  it  reflects,  soon  be- 
comes heated  and  takes  fire. 

Caroline.  This  is  extremely  curious;  but  why  should 
brown  paper  absorb  more  rays  than  white  paper  ? 

Mrs.  B.  I  am  far  from  being  able  to  give  a  satisfac- 
tory answer  to  that  question.  We  can  form  but  mere 
conjecture  on  this  point ;  and  suppose  that  the  tendency 
to  absorb,  or  reflect  rays,  depends  on  the  arrangement  of 
the  minute  particles  of  the  body,  and  that  this  diversity  of 
arrangement  renders  some  bodies  susceptible  of  reflecting 
one  coloured  ray,  and  absorbing  the  others  ;  whilst  other 
bodies  have  a  tendency  to  reflect  all  the  colours,  and  others 
again,  to  absorb  them  all. 

Emily,  And  how  do  you  know  which  colours  bodies 
have  a  tendency  to  reflect,  or  which  to  absorb  ? 

Mrs.  B.  Because  a  body  always  appears  to  be  of  tne 
colour  which  it  reflects  ;  for  as  we  see  only  by  reflected 
rays,  it  can  appear  but  of  the  colour  of  those  rays. 

Caroline,  But  we  see  all  bodies  of  their  own  natural 
colour,  Mrs,  B, ;  the  grass  and  trees,  green  ;  the  sky, 
blue  ;  the  flowers,  of  various  hues. 

Mrs*  B.  True  :  but  why  is  the  grass  green  ?  because 
it  absorbs  all  except  the  green  rays  ;  it  is  therefore  these 
only  which  the  grass  and  trees  reflect  to  our  eyes,  and 

934.  Why  will  a  piece  of  brown  paper  placed  beneath  a  lens, 
which  collects  the  sun's  rays,  take  fire  sooner  than  a  piece  of  white 

paper  r 935.     What  conjecture  is  given  for  the  brown  paper's 

absorbing  more  rays  than  the  white  ? 936.     How  do  we  know 

which  colours  bodies  have  a  tendency  to  reflect,  and  which  to  ab^ 
sorb  ? 


ON  REFRACTION  AND  COLOURS.  '22'S 

^hich  makes  them  appear  green.  The  sky  and  flowers, 
in  the  same  manner,  reflect  the  various  colours  of  which 
they  appear  to  us  ;  the  rose,  the  red  rays ;  the  violet,  the 
blue  ;  the  jonquil,  the  yellow,  &lc. 

Caroline.  But  these  are  the  permanent  colours  of  the 
grass  and  flowers,  whether  the  sun's  rays  shine  on  them 
or  not. 

Mrs,  B.  Whenever  you  see  those  colours,  the  flowers 
must  be  illumined  by  some  light ;  and  light,  from  what- 
ever source  it  proceeds,  is  of  the  same  nature,  composed 
of  the  various  coloured  rays,  which  paint  the  grass,  the 
flowers,  and  every  coloured  object  in  nature. 

Caroline,  But,  Mrs.  B.,  the  grass  is  green,  and  the 
flowers  are  coloured,  whether  in  the  dark  or  exposed  to 
the  light  ? 

Mrs,  B,     Why  should  you  think  so  ? 
Caroline,     It  cannot  be  otherwise. 
Mrs,  B,     A  most  philosophical  reason  indeed !  Butj 
as  I  never  saw  them  in  the  dark,  you  will  allow  me  to  dis- 
sent from  your  opinion. 

Caroline,  What  colour  do  you  suppose  them  to  be. 
then,  in  the  dark  ? 

Mrs,  B,  None  at  all ;  or  black,  which  is  the  same 
thing.  You  can  never  see  objects  without  light.  Light 
is  composed  of  colours,  therefore  there  can  be  no  light 
without  colours ;  and  though  every  object  is  black,  or 
without  colour  in  the  dark,  it  becomes  coloured,  as  soon 
as  it  becomes  visible.  It  is  visible,  indeed,  but  by  the 
coloured  rays  which  it  reflects  ;  therefore  we  can  see  it 
only  when  coloured. 

Caroline,  All  you  say  seems  very  true,  and  I  know  not 
what  to  object  to  it ;  yet  it  appears  at  the  same  time  in- 
credible !  What,  Mrs.  B.,  are  we  all  as  black  as  negroes, 
in  the  dark  ?  you  make  me  shudder  at  the  thought. 

Mrs,  B,  Your  vanity  need  not  be  alarmed  at  the  idea, 
as  you  are  certain  of  never  being  seen  in  that  state. 

Caroline,  That  is  some  consolation,  umioubtedly  ; 
but  what  a  melancholy  reflection  it  is,  that  all  nature 
which  appears  so  beautifully  diversified  with  colours 
should  be  one  uniform  mass  of  blackness  ! 

Mrs,  B,  Is  nature  less  pleasing  for  being  coloured,  as 
well  as  illumined  by  the  rays  of  light ;  and  are  colours  less 

937      Are  colours  essential  properties  of  bodies  ? -938.      On. 

what  do  they  depend  ? 939.     What  colour  do  objects^  have  in 

the  dark  ? 


224  ON  REFRACTION  AND  COLOURS. 

beautiful  for  being  accidental,  rather  than  essential  pro- 
perties of  bodies  ? 

Providence  appears  to  have  decorated  nature  with  the 
enchanting  diversity  of  colours  which  we  so  much  admire, 
for  the  sole  purpose  of  beautifying  the  scene,  and  render- 
ing it  a  source  of  pleasurable  enjoyment :  it  is  an  orna- 
ment which  embellishes  nature  whenever  we  behold  her. 
What  reason  is  there  to  regret  that  she  does  not  wear  it 
when  she  is  invisible  ? 

Emily,  I  confess,  Mrs.  B.,  that  I  have  had  my  doubts 
as  well  as  Caroline,  though  she  has  spared  me  the  pains 
of  expressing  them  ;  but  I  have  just  thought  of  an  experi- 
ment, which,  if  it  succeeds,  w^ill,  I  am  sure,  satisfy  us 
both.  It  is  certain,  that  we  cannot  see  bodies  in  the  dark, 
to  know  whether  they  have  any  colour.  But  we  may  place 
a  coloured  body  in  a  ray  of  light,  which  has  been  refracted 
by  a  prism  ;  and  if  your  theory  is  true,  the  body,  of  what- 
ever colour  it  naturally  is,  must  appear  of  the  colour  of  the 
ray  in  which  it  is  placed  ;  for  since  it  receives  no  other 
coloured  rays,  it  can  reflect  no  others. 

Caroline,  Oh  !  that  is  an  excellent  thought,  Emily  ; 
will  you  stand  the  test,  Mrs.  B.  ? 

Mrs,  B,  I  consent  :  but  we  must  darken  the  room, 
and  admit  only  the  ray  which  is  to  be  refracted ;  other- 
wise, the  white  rays  will  be  reflected  on  the  body  under 
trial  from  various  parts  of  the  room.  With  what  do  you 
choose  to  make  the  experiment  ? 

Caroline,  This  rose  :  look  at  it,  Mrs.  B.,  and  tell  me 
whether  it  is  possible  to  deprive  it  of  its  beautiful  colour? 

Mrs,  B,  We  shall  see. — I  expose  it  first  to  the  red 
rays,  and  the  flowT.r  appears  of  a  more  brilliant  hue ;  but 
observe  the  green  leaves — 

Caroline,  They  appear  neither  red  nor  green  ;  but  of 
a  dingy  brown  with  a  reddish  glow  ! 

Mrs,  B,  They  cannot  be  green,  because  they  have  no 
green  rays  to  reflect ;  neither  are  they  red,  because 
green  bodies  absorb  most  of  the  red  rays.  But  though 
bodies,  from  the  arrangement  of  their  particles,  have  a 
tendency  to  absorb  some  rays,  and  reflect  others,  yet  it 

940.     What  experiment  is  proposed  to  prove  that  bodies  appear 

of  the  colour  of  the  particular  ray  in  which  they  are  placed? 

941.  Why  is  it  necessary  to  darken  the  room  in  which  the  experi- 
ment is  to'^be  made  ? 942.     How  would  a  green  object  appear 

placed  in  a  red  ray  ? 


ON  REFRACTION  AND  COLOURS.  225 

is  not  natural  to  suppose,  that  bodies  are  so  perfectly  uni- 
form in  their  arrangement,  as  to  reflect  only  pure  rays  of 
one  colour,  and  perfectly  absorb  the  others  ;  it  is  found, 
on  the  contrary,  that  a  body  reflects,  in  great  abundance, 
the  rays  which  determine  its  colour,  and  the  others  m  a 
greater  or  less  degree,  in  proportion  as  they  are  nearer  or 
further  from  its  own  colour,  in  the  order  of  refrangibility. 
The  green  leaves  of  the  rose,  therefore,  will  reflect  a  few 
of  the  red  rays  which,  blended  with  their  natural  black- 
ness, give  them  that  brown  tinge  ;  if  they  reflected  none 
of  the  red  rays,  they  would  appear  perfectly  black.  Now 
I  shall  hold  the  rose  in  the  blue  rays — 

Caroline,  Oh,  Emily,  Mrs.  B.  is  right !  look  at  the 
rose  :  it  is  no  longer  red,  but  of  a  dingy  blue  colour,-  - 

Emily.  This  is  the  most  wonderful  of  any  thing  we 
have  yet  learned.  But,  Mrs.  B.,  what  is  the  reason  that  the 
green  leaves  are  of  a  brighter  blue  than  the  rose  ? 

Mrs,  B,  The  green  leaves  reflect  both  blue  and  yel- 
low rays,  which  produces  a  green  colour.  They  are  now 
in  a  coloured  ray,  which  they  have  a  tendency  to  reflect ; 
they,  therefore,  reflect  more  of  the  blue  rays  than  the  rose, 
(which  naturally  absorbs  that  colour,)  and  will,  of  course, 
appear  of  a  brighter  blue. 

Emily,  Yet,  in  passing  the  rose  through  the  different 
colours  of  the  spectrum,  the  flower  takes  them  more  rea- 
dily than  the  leaves. 

Mrs,  B,  Because  the  flower  is  of  a  paler  hue.  Bodies 
which  reflect  all  the  rays  are  white ;  those  which  absorb 
them  all  are  black  :  between  these  extremes,  the  body 
appears  lighter  or  darker,  in  proportion  to  the  quantity  of 
rays  they  reflect  or  absorb.  This  rose  is  of  a  pale  red  : 
it  approaches  nearer  to  white  than  black  ;  it  therefore  re- 
flects rays  more  abundantly  than  it  absorbs  them. 

Emily,  But  if  a  rose  has  so  strong  a  tendency  to  re- 
flect rays,  I  should  have  imagined  that  it  would  be  of  a 
deep  red  colour. 

943.     Why  would  it  appear  of  a  brownish  tinge  P 944.     If 

a  red  object  be  placed  in  a  blue  ray  how  will  it  appear  ? 945. 

Why  does  an  object  that  is  green  placed  in  a  blue  ray  appear  of 
a  brighter  blue,  than  an  object  that  is  red  when  placed  in  the  same 

coloured  ray  ? 946.     In  passing  a  red  and  green  object  through 

the  different  colours  of  the  spectrum,  why  does  the  red  one  take 
them  more  readily  than  the  green  one  .' 947.  What  bodies  re- 
flect all  the  rays  that  fall  on  them  ' 948,     What  ones  absorb 

them  ? 


226  ON  REFRACTION  AND  COLOURS. 

Mrs,  B.  I  mean  to  say,  that  it  has  a  general  tenden- 
cy to  reflect  rays.  Pale  coloured  bodies  reflect  all  the  co- 
loured rays  to  a  certain  degree,  which  produces  their  pale- 
ness, approaching  to  whiteness  ;  but  one  colour  they  re- 
flect more  than  the  rest ;  this  predominates  over  the  white, 
and  determines  the  colour  of  the  body.  Since,  then,  bo- 
dies of  a  pale  colour  in  some -degree  reflect  all  the  rays  of 
light,  in  passing  through  the  various  colours  of  the  spec- 
trum, they  will  reflect  them  all  with  tolerable  brilliancy  ; 
but  will  appear  most  vivid  in  the  ray  of  their  natural  co- 
lour. The  green  leaves,  on  the  contrary,  are  of  a  dark 
colour,  bearing  a  stronger  resemblance  to  black,  than  to 
white ;  they  have,  therefore,  a  greater  tendency  to  ab- 
sorb, than  to  reflect  rays ;  and  reflecting  very  few  of  any 
but  the  blue  and  yellow  rays,  they  will  appear  dingy  in 
passing  through  the  other  colours  of  the  spectrum. 

Caroline,  They  must,  however,  reflect  great  quan- 
tities of  the  green  rays  to   produce  so  deep  a  colour. 

Mrs,  B,  Deepness  or  darkness  of  colour  proceeds 
rather  from  a  deficiency  than  an  abundance  of  reflected 
rays.  Remember  that  bodies  are,  of  themselves,  black ; 
and  if  a  body  reflects  only  a  few  green  rays,  it  will  appear 
of  a  dark  green  ;  it  is  the  brightness  and  intensity  of  the 
colour  which  show  that  a  great  quantity  of  rays  are  re- 
flected. 

Emily,  A  white  body,  then,  which  reflects  all  the 
rays,  will  appear  equally  bright  in  all  the  colours  of  the 
spectrum. 

Mrs,  B,  Certainly  ;  and  this  is  easily  proved  by  pass- 
ing a  sheet  of  white  paper  through  the  rays  of  the  spec- 
trum. 

Caroline.  What  is  the  reason  that  blue  often  appears 
green  by  candle-light  ? 

Mrs,  B,  The  light  of  a  candle  is  not  so  pure  as  that 
of  the  sun  ;  it  has  a  yellowish  tinge,  and  when  refracted 
by  the  prism,  the  yellow  rays  predominate  ;  and  as  blue 
bodies  reflect  the  yellow  rays  in  the  next  proportion, 
(being  next  in  order  of  refrangibility,)  the  superabundance 
of  yellow  rays  gives  to  blue  bodies  a  greenish  hue. 

Caroline,  Candle-light  must  then  give  to  all  bodies  a 
yellowish  tinge,  from  the  excess  of  yellow  rays  ;  and  yet 

949      To  what  is  darkness  of  colour  owing  ? 950.     What  is 

the  reason  the  blue   often  appears  green  by  candle-light  ? 951. 

Why  do  persons  of  a  sallow  complexion   appear  fairer   or  whiter 
by  night,  if  the  candle-light  ^ives  all  objects  a  yellowish  tinge  - 


ex\  REFRACTION  AND  COLOUR.  227 

it  is  a  common  remark,  that  people  of  a  sallow  complexion 
appear  fairer  or  whiter  by  candle-light. 

Mrs.  B,     The  yellovv  cast  of  their  complexion  is  not 
so  striking,  when  every  object  has  a  yellovv  tinge. 
Emily,  Pray,  why  does  the  sun  appear  red  through  a  fog  ? 

Mrs,  B.  It  is  supposed  to  be  owing  to  the  red  rays  hav- 
ing a  greater  momentum,  which  gives  them  power  to  tra- 
verse so  dense  an  atmosphere.  For  the  same  reason,  the 
sun  generally  appears  red  at  rising  and  sitting  ;  as  the 
increased  quantity  of  atmosphere,  which  the  oblique  rays 
must  traverse,  loaded  with  the  mists  and  vapours  which 
are  usually  formed  at  those  times  prevents  the  other  rays 
from  reaching  us. 

Caroline,  And,  pray,  why  are  the  skies  of  a  blue  co- 
lour. 

3Irs,  B,  You  should  rather  say,  the  atmosphere  ;  for 
the  sky  is  a  very  vague  term,  the  meaning  of  which  it 
would  be  difficult  to  dehne  philosophically. 

Caroline,  But  the  colour  of  the  atmosphere  should  be 
white,  since  all  the  rays  traverse  it  in  their  passage  to  the 
earth. 

3Irs,  B,  Do  not  forget  that  we  see  none  of  the  rays 
which  pass  from  the  sun  to  the  earth,  excepting  those 
which  meet  our  eyes  ;  and  this  happens  only  if  we  look 
at  the  sun,  and  thus  intercept  the  rays,  in  which  case,  you 
know,  the  sun  appears  white.  The  atmosphere  is  a  trans- 
parent medium,  through  which  the  sun's  rays  pass  freely 
to  the  earth  ;  but  when  reflected  back  into  the  atm.osphere, 
their  momentum  is  considerably  diminished  ;  and  they 
have  not  all  of  them  power  to  traverse  it  a  second  time. 
The  momentum  of  the  blue  rays  is  least ;  these,  there- 
fore, are  the  most  impeded  in  their  return,  and  are  chiefly 
reflacted  by  the  atmosphere  :  this  reflection  is  performed 
in  every  possible  direction  ;  so  that  whenever  we  look  at 
the  atmosphere,  some  of  these  rays  fall  upon  our  eyes  ; 
hence  we  see  the  air  of  a  blue  colour.  If  the  atmosphere 
did  not  reflect  any  rays,  though  the  objects  on  the  sur- 
face of  the  earth  would  be  illumined,  the  skies  would  ap- 
pear perfectly  black. 

Cdroline,  Oh,  how  melancholy  that  would  be ;  and 
how  pernicious  to   the   sight,  to   be   constantly  viewing 

052.     Why  does  tlie  san  appear  red  in  the  morninfr  and  wh'^n 

seon  throuirh  ibfr  or  clonds  ? 053.     Why  does   the  sky  or  at- 

ino3phere  appear  blue  ? 054.     How  would  the  sky  appear  if  tho 

atmosphere  refiec.led  none  of  the  rays  of  light  ? 


^28  ON  REFRACTION  AND  COLOURS. 

bright  objects  against  a  black  sky  !  But  what  is  the  reason 
that  bu^K^s  often  change  their  colour;  as  leaves  which 
wither  in  autumn,  or  a  spot  of  ink  which  produces  an  iron- 
mould  on  linen  ? 

Mrs,  B,  It  arises  from  some  chemical  change,  which 
takes  place  in  the  internal  arrangement  of  the  parts,  by 
which  they  lose  their  tendency  to  reflect  certain  colours, 
and  acquire  the  power  of  reflecting  others.  A  withered 
leaf  thus  no  1  »iiger  reflects  the  blue  rays  ;  it  appears, 
therefore,  yellow,  or  has  a  slight  tendency  to  reflect  sevc' 
ral  rays  which  produce  a  dingy  brown  colour. 

An  ink-spot  on  linen  at  first  absorbs  all  the  rays  ;  bu^ 
exposed  to  the  air,  it  undergoes  a  chemical  change,  anc 
the  spot  partially  regains  its  tendency  to  reflect  colours, 
but  with  a  preference  to  reflect  the  yellow  rays,  and  such, 
is  the  colour  of  the  iron-mould. 

Emily.  Bodies,  then,  far  from  being  of  the  colour 
which  they  appear  to  possess,  are  of  that  colour  which 
they  have  the  greatest  aversion  to,  which  they  will  not  in- 
corporate with,  but  reject  and  drive  from  them. 

Mrs,  B,  It  certainly  is  so ;  though  I  scarcely  dare 
venture  to  advance  such  an  opinion  whilst  Caroline  is  con- 
templating her  beautiful  rose. 

Caroline.  My  poor  rose  !  you  are  are  not  satisfied  with 
<lepriving  it  of  colour,  but  even  make  it  have  an  aversion 
to  it ;  and  I  am  unable  to  contradict  you. 

Emihj.  Since  dark  bodies  absorb  more  solar  rays  than 
light  ones,  the  former  should  sooner  be  heated  if  exposed 
to  the  sun. 

Mrs.  B.  And  they  are  found  by  experience  to  be  so. 
Have  you  never  observed  a  black  dre^i  to  be  warmer  than  a 
white  one  ? 

Emily.  Yes,  and  a  white  one  more  dazzling  :  the 
black  is  heated  by  absorbing  the  rays,  the  white  dazzling 
by  reflecting  them. 

Caroline.  And  this  was  the  reason  that  the  brown  paper 
was  burnt  in  the  focus  of  the  lens,  whilst  the  white  paper 
exhibited  the  most  luminous  spot,  but  did  not  take  fire. 

Mrs.  B.  It  was  so.  It  is  now  full  time  to  conclude 
our  lesson.  At  our  next  meeting,  I  shall  give  you  a  de- 
scription of  the  eye. 

955.     Whnt  is  the  reason  that  they  often  change  their  colour  ? 

956      What  dress  is  warmest,  a  black  or  a  white  on.*  ? 957. 

Why  is  a  black  one  warmest  ? 958.     Why  is  a  white  raor*.^  daz- 

/^ling  than  a  black  drpss  ? 


opTicKs.  229 

CONVERSATION  XVlI. 

OPTICKS. 

ON  THE  STRUCTURE  OF  THE  EYE,  AND  OPTICAL  INSTRU- 
MENTS. 

Description  of  the  Eye ;  Of  the  Image  on  the  Retina ; 
Refraction  of  the  Humours  of  the  Eye  ;  Of  the  Use  of 
Spectacles  ;  Of  the  Single  Microscope ;  Of  the  Double 
Microscope  ;  Of  the  Solar  Microscope  ;  Magick  EaU" 
tern ;  Refracting   Telescope ;  Refecting  Telescope* 

MRS.    B. 

The  body  of  the  eye  is  of  a  spherical  form  :  (fig.  t, 
plate  XXI.)  It  has  two  membraneous  coverings  ;  the  exter- 
nal one,  a  a  a,  is  called  the  sclerotica ;  this  has  a  projec- 
tion in  that  part  of  the  eye  which  is  exposed  to  view,  b  b, 
which  is  called  the  cornea,  because,  when  dried,  it  has 
nearly  the  consistence  of  very  fine  horn,  and  is  sufficient- 
ly transparent  for  the  light  to  obtain  free  passage  through  it. 

The  second  membrane  which  lines  the  cornea,  and  en- 
velopes the  eye,  is  called  the  choroid,  c  c  c ;  this  has  an 
opening  in  front,  just  beneath  the  cornea,  which  forms  the 
pupil,  cl  d,  through  which  the  rays  of  light  pass  into  the 
€ye.  The  pupil  is  surrounded  by  a  coloured  border,  call- 
ed the  iris,  c  e,  which,  by  its  muscular  motion,  always  pre- 
serves the  pupil  of  a  circular  form,  whether  it  is  expanded 
in  the  dark,  or  contracted  by  a  strong  light.  This  you 
will  understand  better  by  examining  fig.  2. 

Emily.  I  did  not  know  that  the  pupil  was  susceptible 
of  varying  its  dimensions. 

Mrs,  B.  The  construction  of  the  eye  is  so  admirable, 
that  it  is  capable  of  adapting  itself,  more  or  less,  to  the 
circumstances  in  which  it  is  placed.     In  a  faint  light  tJie 

959.     What  is  the  form  of  the  body  of  the  eye  ? 960.  Which 

figure  represents  an  eye  ? 961.     What  is  the  external  covjering 

of  the  eye  called  ? 962.     Which  part  of  the  eye   is  called  the 

cornea.^ 963.     From  what  does  the  cornea  take  its  name  ? 

964.     What  part  of  the  eye  is  called  the  choroid  ? 965.     What 

part  of  the  figure  represents  the  choroid  ? 966.     What  is  that 

part  of  the  eye  called  through  which  the   light  passes  ? 967. 

By  what  part   of  the  figure  is  the  pupil  represented  ? ^968. 

By  what  is  the  pupil  of  the  eye  surrounded  ? 969.  What  repre- 
sents the  iris  in  the  figure  ? 970.     Is  the  pupil  of  the  eye  al- 
ways of  the  same  size  ? 
20 


•230  OPTICKS. 

pupil  dilates  so  as  to  receive  an  additional  quantity  of  rays, 
and  in  a  strong  liglU  it  contracts,  in  order  to  prevent  the 
intensity  of  the  light  from  injuring  the  optick  nerve. 
Observe  Emily's  eyes,  as  she  sits  lookuig  towards  the  win- 
dows ;  her  pupils  appear  very  small,  and  the  iris  large. 
Now,  Emily,  turn  from  the  light  and  cover  your  eyes  with 
your  hand,  so  as  entirely  to  exclude  it  for  a  few  moments. 

Caroline.  How  very  much  the  pupils  of  licr  eyes  are 
now  enlarged,  and  the  iris  diminished.  This  is,  no  doubt, 
the  reason  why  the  eyes  sufterpain,  when  from  darkness 
they  suddenly  come  into  a  strong  light ;  for  the  pupil  be- 
ing dilated,  a  quantity  of  rays  must  rush  in  before  it  has 
time  to  contract. 

Emily.  And  when  we  go  from  a  strong  light  into  ob- 
scurity, we  at  first  imagine  ourselves  in  total  darkness ; 
for  a  sufficient  number  of  rays  cannot  gain  admittance 
into  the  contracted  pupil,  to  enable  us  to  distinguish  ob- 
jects :  but  in  a  few  minutes  it  dilates,  and  we  clearly  per- 
ceive objects  which  were  before  invisible. 

Mrs.  B.  It  is  just  so.  The  choroid  c  c/\s  imbued 
with  a  black  liquor  which  serves  to  absorb  all  the  rays 
that  are  irregularly  retkcted,  and  to  convert  the  body  of 
ihe  eye  into  a  more  perfect  camera  obscura.  When  the 
pupil  is  expanded  to  its  utmost  extent,  it  is  capable  of  ad- 
mitting ten  times  the  quantity  of  light  that  it  does  when 
most  contracted.  In  cats,  and  animals  which  are  said  to 
see  in  the  dark,  the  power  of  dilatation  and  contraction 
of  the  pupil  is  still  greater  ;  it  ijB  computed  that  their  pu- 
pils may  receive  one  hundred  times  more  light  at  one 
time  than  at  another. 

Within  these  coverings  of  the  eye-ball  are  contained 
three  transparent  substances,  called  humours.     The  first 


971.     When  is  it  dilated,  and  when  contracted  r 972.     \Miy 

does  it  give  the  eyes  pain  on  tirst  going  into  a  bright  hght  from  a 
dark  room  r 073.  Why  does  it  seem  much  darker  on  first  go- 
ing out  in  the  night,  than  after  we   have  been  out   a  short  time  ? 

974.     How   much  more  light  is  admitted   when  the  pupil  is 

extended  to  the  utmost,   than  when   niost   contracted' 975. 

Why  can  cats,  horses,  and  some  other  animals,  see  better  in  the 

night  than  we  can 076.     How  much  is  it  thought  the  pupil 

of  their  eyes  extend  and  contract  r 977.     What  is  contained 

"within  the  coverings  of  the  eve-ball  ' 


OPTICKS.  231 

occupies  the  space  immediately  behind  the  cornea,  and  is 
called  the  aqueous  humour,  y/,  from  its  liquidity  and  its 
resemblance  to  water.  Beyond  this  is  situated  the  crys- 
talline humour,  g  g,  so  called  from  its  clearness  and  trans- 
parency :  it  has  the  form  of  a  lens,  and  refracts  the  rays  of 
light  in  a  greater  degree  of  perfection  than  any  that  have 
been  constructed  by  art :  it  is  attached  by  two  muscles. 
m  m,  to  each  side  of  the  choroid.  The  back  part  of  the 
eye,  between  the  crystalline  humour  and  the  retina,  is  fill- 
ed by  the  vitreous  humour,  h  Ji,  which  derives  its  name 
from  a  resemblance  it  is  supposed  to  bear  to  glass  or  vi-* 
trifled  substances. 

The  membraneous  coverings  of  the  eye  are  intended 
chiefly  for  the  preservation  of  the  retina,  i  ?*,  which  is  by 
far  the  most  important  part  of  the  eye,  as  it  is  that  which 
receives  the  impression  of  the  objects  of  sight,  and  con- 
veys it  to  the  mind.  The  retina  consists  of  an  expansion 
of  the  optick  nerve,  of  a  most  perfect  whiteness  :  it  pro- 
ceeds from  the  brain,  enters  the  eye,  at  n,  on  the  side  next 
the  nose,  and  is  finely  spread  over  the  interiour  surface 
of  the  choroid. 

The  rays  of  light  which  enter  the  eye  by  the  pupil  are 
refracted  by  the  several  humours  in  their  passage  through 
them,  and  unite  in  a  focus  on  the  retina. 

Caroline.  I  do  not  understand  the  use  of  these  refract- 
ing humours ;  the  image  of  objects  is  represented  in  the 
camera  obscura,  without  any  such  assistance. 

Mrs.  B.  That  is  true  ;  but  the  representation  would 
be  much  more  strong  and  distinct,  if  we  enlarge  the  open- 
ing of  the  camera  obscura,  and  received  the  rays  into  it 
through  a  lens. 

I  have  told  you  that  rays  proceed  from  bodies  in  all 
possible  directions.  We  must,  therefore,  consider  every 
part  of  an  object  which  sends  rays  to  our  eyes,  as  points 
from  which  the  rays  diverge,  as  from  a  centre. 

978.     What    are   the   three    humours  called  ? 979.      From 

what  does  the  aqueous  humour  derive  its  name  ? — !— 980.     From 

what  does  the  crystalline  humour  derive  its  name  ? 981.    From 

what  does  the  vitreous  humour  derive  its  name  ? 982.  For  what 

are  the  membraneous  coverings  of  the  eye  chiefly  intended .' 

983.     Which  part  of  the  figure  exhibits  the  retina  > 984.     Of 

what  does  the   retina  consist  ^ — — 985.     How  is  the  light  which 

enters  the  pupil  affected  by  the  several  humours  ? 986.     What 

would  be  the  consequence  if  the  light  admitted  by  the  pupil  were 
not  refracted  bv  the  humours  ? 


232  opTrcKs^ 

Emily,  These  divergent  rays,  issuing  from  a  single 
point,  I  believe  you  told  us,  were  called  a  pencil  of  rays  ? 

Mrs,  jB.  Yes.  Now,  divergent  rays,  on  entering  the 
pupil,  do  not  cross  each  other  ;  the  pupil,  however,  is 
sufficiently  large  to  admit  a  small  pencil  of  them  ;  and 
these,  if  not  refracted  to  a  focus  by  the  humours,  would 
continue  diverging  after  they  had  passed  the  pupil,  would 
fall  dispersed  upon  the  retina^  and  thus  the  image  of  sl 
single  point  would  be  expanded  over  a  large  portion  of 
the  retina.  The  divergent  rays  from  every  other  point 
of  the  object  would  be  spread  over  a  similar  extent  of 
space,  and  would  interfere  and  be  confounded  with  the 
first  ;  so  that  no  distinct  image  could  be  formed,  and  the 
retina  would  represent  total  confusion  both  of  figure  and 
colour.  Fig.  3  represents  two  pencils  of  rays  issuing 
from  two  points  of  the  tree  A  B,  and  entering  the  pupil  C, 
refracted  by  the  crystalline  humour  D,  and  forming  dis- 
tinct images  of  the  spot  they  proceed  from,  on  the  retina 
at  a  h.  Fig.  4  differs  from  the  preceding,  merely  from 
not  being  supplied  with  a  lens  ;  in  consequence  of  which 
the  pencils  of  rays  are  not  refracted  to  a  focus,  and  no 
distinct  image  is  formed  on  the  retina.  I  have  delineated 
only  the  rays  issuing  from  two  points  of  an  object,  and 
distinguished  the  two  pencils  in  fig.  4,  by  describing  one 
of  them  with  dotted  lines  ;  the  interference  of  these  two 
pencils  of  rays  on  the  retina  will  enable  you  to  form  an 
idea  of  the  confusion  which  would  arise,  from  thousands 
and  millions  of  points  at  the  same  instant  pouring  their 
divergent  rays  upon  the  retina. 

Emily,  True  ;  but  I  do  not  yet  well  understand  how 
the  refracting  humours  remedy  this  imperfection, 

Mrs.  B.  The  refraction  of  these  several  humours 
unite  the  whole  of  a  pencil  of  rays,  proceeding  from  any 
one  point  of  an  object,  to  a  corresponding  point  on  the 
retina,  and  the  image  is  thus  rendered  distinct  and  strong. 
If  you  conceive,  in  fig.  3,  every  point  of  the  tree  to  send 
forth  a  pencil  of  rays  similar  to  those,  A  B,  every  part  of 
the  tree  will  be  as  accurately  represented  on  the  retina  as 
the  points  a  h. 

Emily,  How  admirably,  how  wonderfully,  this  is  con- 
trived ! 

987.     What  does  Fi^.  3.  plate  XXI.  represent  ? 98S;'~\Vhat 

does  Fig.  4.  of  that  plate  represent  ? 989.     How  does  the  re- 

fraclinor  hnmonr  romodvthp  drferts  exlubited  inthatfi<^nre  ' 


OPTICKS.  233 

Caroline.  But  since  the  eye  requires  refracting  hu- 
mours in  order  to  have  a  distinct  representation  formed 
on  the  retina,  why  is  not  the  same  refraction  necessary 
for  the  image  formed  in  the  camera  obscura  ? 

Mrs,  jB.  Because  the  aperture  through  which  we  re- 
ceived the  rays  into  the  camera  obscura  is  extremely 
small ;  so  that  but  very  ^e\v  of  the  rays  diverging  from  a 
point,  gain  admittance  ;  but  we  will  now  enlarge  the 
aperture,  and  furnish  it  with  a  lens,  and  you  will  find  the 
landscape  to  be  more  perfectly  represented. 
Caroline,  How  obscure  and  confused  the  image  is  now  that 
you  have  enlarged  the  opening,  without  putting  in  the  lens  ! 

Mrs-,  B,  Such  or  very  similar  would  be  the  representa- 
tion on  the  retina,  unassisted  by  the  refracting  humours. 
But  see  what  a  difference  is  produced  by  the  introduction 
of  the  lens,  which  collects  each  peacil  of  divergent  rays 
into  their  several  foci. 

Caroline,  The  alteration  is  wonderful :  the  represen- 
tation is  more  clear,  vivid,  and  beautiful  than  ever. 

3Irs,  B,  You  will  now  be  able  to  understand  the  na- 
ture of  that  imperfection  of  sight,  which  arises  from  the  eyes 
being  too  prominent.  In  such  cases,  the  crystalline  humour, 
D,  (fig.  5.)  being  extremely  convex,  refracts  the  rays  too 
much,  and  collects  a  pencil,  proceeding  from  the  object  A 
B,  into  a  focus,  F,  before  they  reach  the  retina.  From  this 
focus  the  rays  proceed  diverging,  and  consequently  form  a 
very  confused  image  on  the  retina  at  a  h.  This  is  the  de- 
fect of  short-sighted  people  ? 

Emily,  I  understand  it  perfectly.  But  why  is  this 
defect  remedied  by  bringing  the  object  nearer  to  the  eye, 
as  we  find  to  be  the  case  with  short-sighted  peopk  ? 

Mrs.  B.  The  nearer  you  bring  an  object  to  your  eye, 
the  more  divergent  the  rays  fall  upon  the  crystalline  hu- 
mour, and  they  are  consequently  not  so  soon  converged  to 
a  focus  ;  this  focus,  therefore,  either  falls  upon  the  retina, 
or  at  least  approaches  nearer  to  it,  and  the  object  is  pro- 
portionally distinct,  as  in  fig.  6. 

Emily,  The  nearer,  then,  you  bring  an  object  to  a 
lens,  the  further  the  image  recedes  behind  it. 

990.  Why  is  not  something  like  the  refiracting  humours  neces- 
sary in  the  camera  obscura  ? 991.     What  peculiarity  of  the  eye 

causes  some  persons  to  be  short-sighted  ? 992.      Which  figure 

represents  the  eye  of  a  short-sighted  person  ? 993.     Why  can 

short-sighted  persons  see  better  by  bringing  the  objects  near  to 

the  eye  ? 994.    By  which  figure  is  this  illustrated  ? 

20  *♦ 


234  tfPTICKs. 

Airs,  B,  Certainly.  But  short-sighted  persons  have 
another  resource  for  objects  which  they  cannot  approach 
to  their  eyes ;  this  is  to  place  a  concave  lens,  C  D,  (fig. 
1,  plate  XXII.)  before  the  eye,  in  order  to  increase  the  di- 
vergence of  the  rays.  The  effect  of  a  concave  lens  is, 
you  know,  exactly  the  reverse  of  a  convex  one  :  it  renders 
parallel  rays  divergent,  and  those  which  are  already  diver- 
gent, still  more  so.  By  the  assistance  of  such  glasses, 
therefore,  the  rays  from  a  distant  object  fail  on  the  pupil, 
as  divergent  as  those  from  a  less  distant  object ;  and, 
with  short-sighted  people,  they  throw  the  image  of  a  dis- 
tant object  back  as  far  as  the  retina. 

Caroline,     This  is  an  excellent  contrivance,  indeed. 

Mrs,  B,  And  tell  me,  what  remedy  would  you  devise 
for  such  persons  as  have  a  contrary  defect  in  their  sight ; 
that  is  to  say,  in  whom  the  crystalline  humour,  being  too 
flat,  does  not  refract  the  rays  sufficiently,  so  that  they 
reaQh  the  retina  before  they  are  converged  to  a  point  1 

Caroline,  I  suppose  that  a  contrary  remedy  must  be 
applied  to  this  defect ;  that  is  to  say,  a  convex  lens,  L  M, 
fig.  2.  to  make  up  for  the  deficiency  of  convexity  of  the 
crystalline  humour  O  P.  For  the  convex  lens  would 
bring  the  rays  nearer  together,  so  that  they  would  fall 
either  less  divergent,  or  parallel  on  the  crystalline  humour  ; 
and,  by  being  sooner  converged  to  a  focus,  would  fall  on 
the  retina. 

i>/?*5.  B,  Very  well,  Caroline.  This  is  the  reason 
why  elderly  people,  the  humours  of  whose  eyes  are  decay- 
ed by  age,  are  under  the  necessity  of  using  convex  specta- 
cles. And  when  deprived  of  that  resource,  they  hold  the 
object  at  a  distance  from  their  eyes,  as  in  fig.  4.,  in  order 
to  bring  the  focus  forwarder. 

Caroline,  I  have  often  been  surprised,  when  my 
grandfather  reads  without  his  spectacles,  to  see  him  hold 
the  book  at  a  considerable  distance  from  his  eyes.  But  I 
now  understand  it ;  for  the  more  distant  the  object  is 
from  the  crystalline,  the  nearer  the  image  will  be  to  it. 

995.  What  other  resouree  have  short-sighted  persons,  for  reme- 
dying the  defect  of  their  eyes  ? 996.    Why  will  a  concave  lens 

remedy  this  eifect  ? 997.     What  is  the  design  of  Fig.  1,  plate 

XXII.  ? 998.     What  is  the  reason  that  elderly  people  usually 

lose  their  sight  ? — ^ — 999.     What  remedy  is  there  for  the  eyes  when 

the  humours  are  decayed  or  flattened  ^ 1000.     Which  figure 

illustrates  this? 1001.      Why  do   old  people  without  convex 

glasses  hold  the  objects  to  be  seen  at  a  distance  fiom  the  eye.'' 


oPTicKs.  235 

Emily.  I  comprehend  the  nature  of  these  two  oppo- 
site defects  very  well ;  but  I  cannot  now  conceive,  how 
any  sight  can  be  perfect :  for  if  the  crystalline  humour  is 
of  a  proper  degree  of  convexity,  to  bring  the  image  of  dis- 
tant objects  to  a  focus  on  the  retina,  it  will  not  represent 
near  objects  distinctly  ;  and  if,  on  the  contrary,  it  is  adapt- 
ed to  give  a  clear  image  of  near  objects,  it  will  produce 
a  very  imperfect  one  of  distant  objects. 

Mrs.  JB.  Your  observation  is  very  good,  Emily  :  and 
it  is  true,  that  every  person  would  be  subject  to  one  of 
these  two  defects,  if  we  had  it  not  in  our  power  to  in- 
crease or  diminish  the  convexity  of  the  crystalline  humour, 
and  to  project  it  towards,  or  draw  it  back  from  the  object, 
as  circumstances  require.  In  a  young  well  constructed 
eye,  the  two  muscles  to  which  the  crystalline  humour  is 
attached,  have  so  perfect  a  command  over  it,  that  the  focus 
of  the  rays  constantly  falls  on  the  retina,  and  an  image  is 
formed  equally  distinct  both  of  distant  objects,  and  of 
those  which  are  near. 

Caroline.  In  the  eyes  of  fishes,  which  are  the  only 
eyes  I  have  ever  seen  separate  from  the  head,  the  cornea 
does  not  protrude,  in  that  part  of  the  eye  which  is  exposed 
to  view. 

3Irs.  B.  The  cornea  of  the  eye  of  a  fish  is  not  more 
convex  than  the  rest  of  the  ball  of  the  eye  ;  but  to  supply 
this  deficiency,  thei?  crystalline  humour  is  spherical,  and 
refracts  the  rays  so  much,  that  it  does  not  require  the  as- 
sistance of  the  cornea  to  bring  them  to  a  focus  on  the  re- 
tina. 

Emily.  Pray,  what  is  the  reason  that  we  cannot  see 
an  object  distinctly,  if  we  approach  it  very  near  to  the  eye  1 

Mrs.  B.  Because  the  rays  fall  on  the  crystalline  hu- 
mour too  divergent  to  be  refracted  to  a  focus  on  the  retina ; 
the  confusion,  therefore,  arising  from  viewing  an  object 
too  near  the  eye,  is  similar  to  that  which  proceeds  from  a 
flattened  crystalline  humour ;  the  rays  reach  the  retina  be- 
fore they  are  collected  to  a  focus,  (fig.  4.)  If  it  were  not 
for  this  imperfection,  we  should  be  able  to  see  and  distin- 


1002.     By  what  means  can  the  same  eye  see  distinctly  distant 

objects  and  those  which  are  near  ? 1003.     What  peculiarity  of 

structure  is  there  in  the  eyes  of  fishes  ? 1004.     How  is  this 

seeming  defect  remedied  ? 1005.     What  is  the  reason  that  we 

cannot  sec  an  object  distinctly  when  it  is  placed  very  near  to  the 
eye? — ^1000.     Py  which  figure  is  this  illustrated  I 


236  opTicKs. 

guish  the  parts  of  objects,  which  are  now  invisible  to  us, 
from  their  minuteness  ;  for  could  we  approach  them  very 
near  the  eye,  their  image  on  the  retina  would  be  so  much 
magnified  as  to  render  them  visible. 

Emily,  And  could  there  be  no  contrivance  to  convey 
the  rays  of  objects  viewed  close  to  the  eye,  so  that  they 
should  be  refracted  to  a  focus  on  the  retina. 

Mrs.  B.  The  microscope  is  constructed  for  this  pur- 
pose. The  single  microscope  (fig.  5.)  consists  simply  of 
a  convex  lens,  commonly  called  a  magnifying-glass ;  in 
the  focus  of  which  the  object  is  placed,  and  through  which 
it  is  viewed  :  by  this  means  you  are  enabled  to  approach 
your  eye  very  near  the  object,  for  the  lens,  A  B,  by  di- 
minishing the  divergence  of  the  rays,  before  they  enter 
the  pupil  C,  makes  them  fall  parallel  on  the  crystalline 
humour  D,  by  which  they  are  refracted  to  a  focus  on  the 
retina,  at  R  R. 

JEmihj,  This  is  a  most  admirable  invention,  and  no- 
thing can  be  more  simple,  for  the  lens  magnifies  the  ob- 
ject merely  by  allowing  us  to  bring  it  nearer  to  the  eye. 

Mrs,  B.  Those  lenses,  therefore,  which  have  the 
shortest  focus,  will  magnify  the  object  most,  because  they 
enable  us  to  bring  the  object  nearest  to  the  eye. 

Emily.  But  a  lens,  that  has  the  shortest  focus,  is  most 
bulging  or  convex  ;  and  the  protuberance  of  the  lens  will 
prevent  the  eye  from  approaching  very  near  to  the  object. 

Mrs,  B,  This  is  remedied  by  making  the  lens  ex- 
tremely small  :  it  may  then  be  spherical  without  occupy- 
ing much  space,  and  thus  unite  the  advantages  of  a  short 
focus,  and  of  allowing  the  eye  to  approach  the  object. 

Caroline,  We  have  a  microscope  at  home,  which  is  a 
much  more  complicated  instrument  than  that  you  have 
described, 

Mrs.  B.  It  is  a  double  rarcroscope,  (fig.  6.)  in  which 
you  see  not  the  object  A  B,  but  a  magnified  image  of  it, 
ah.  In  this  microscope,  two  lenses  are  employed,  the 
one  L  M,  for  the  purpose  of  magnifying  the  object,  is 

1007.  In  what  way  can  objects  be  seen  distinctly  when  placed 
near  the  eye? 1008.  Of  what  does  a  single  microscope  con- 
sist?  1009.     What  is  the  object  of  Fig.  5,   plate  XXII. .? 

1010.     What  lenses  will  magnify  objects  most  ? 1011.     What 

kind  of  lenses  has  the  shortest  focus  ? 1012.  What  is  repre- 
sented by  Y\g.  6,  plate  XXII.  ^ 1013.     How  would  you  explaini 

the  use  of  the  double  microscope;  by  the  aid  of  that  figure  ? 


opTicKs.  237 

called  the  object-glass ;  the  other  N  O,  acts  on  the  prin- 
ciple of  the  single  microscope,  and  is  called  the  eye-glass. 

There  is  another  kind  of  microscope,  called  the  solar 
microscope,  which  is  the  most  wonderful  from  its  great 
magnifying  power  ;  in  this  we  also  view  an  image  formed 
by  a  lens,  not  the  object  itself.  As  the  sun  shines,  I  can 
show  you  the  effect  of  this  microscope  :  but  for  this  pur- 
pose, we  must  close  the  shutters,  and  admit  only  a  small 
portion  of  light,  through  the  hole  in  the  window-shutter, 
which  we  used  for  the  camera  obscura.  We  shall  now 
place  the  object  A  B,  (plate  XXIII.  fig.  I.)  which  is  a 
small  insect,  before  the  lens,  C  D,  and  nearly  at  its  fo- 
cus ;  the  image  E  F,  will  then  be  represented  on  the  op- 
posite wall  in  the  same  manner  as  the  landscape  was  in  the 
camera  obscura  ;  with  this  difference,  that  it  will  be  mag- 
nified, instead  of  being  diminished.  I  shall  leave  you  to 
account  for  this,  by  examining  the  figure. 

Emily,  I  see  it  at  once.  The  image  E  F  is  magnified, 
because  it  is  further  from  the  lens,  than  the  object  A  B  ; 
while  the  representation  of  the  landscape  was  diminished 
because  it  was  nearer  the  lens,  than  the  landscape  was. 
A  lens,  then,  answers  the  purpose  equally  well,  either  for 
magnifying  or  diminishing  objects  1 

Mrs,  B.  Yes  ;  if  you  wish  to  magnify  the  image,you 
place  the  object  near  the  focus  of  the  lens  ;  if  you  wish 
to  produce  a  diminished  image,  you  place  the  object  at  a 
distance  from  the  lens,  in  order  that  the  image  may  be 
formed  in  or  near  the  focus. 

Caroline,  The  magnifying  power  of  this  microscope 
is  prodigious,  but  the  indistinctness  of  the  image  for  want 
of  light,  is  a  great  imperfection.  Would  it  not  be  clearer, 
if  the  opening  in  the  shutter  were  enlarged,  so  as  to  ad- 
mit more  light  ? 

Mrs.  B,  If  the  whole  of  the  light  admitted  does  not 
fall  upon  the  object,  the  effect  will  only  be  to  make  the 
room  lighter,  and  the  image  consequently  less  distinct. 

Emily.  But  could  you  not  by  means  of  another  lens 
bring  a  large  pencil  of  rays  to  a  focus  on  the  object,  and 
thus  concentrate  the  whole  of  the  light  admitted  upon  it  1 

1014.  WhatdoesFig.l,  plate  XXTII.  represent? 1015.  How 

would  you  describe  a  solar  microscope  by  the  use  of  this  figure  .'' 

lOlG.     Where  must  an  object  be  placed  in  regard  to  a  lens, 

so  that  the  r.bject  be  magnified  ? 10 i  7.  Where,  so  that  the  ob- 
ject be  diminished  ? 1018.     Where  must  all  the  light  fall,  used 

in  the  solar  microscope,  so  that  the  effect  be  the  most  favourable  ? 


238  opTicKs. 

Mrs,  B.  Very  well.  We  shall  enlarge  the  opening 
and  place  the  lens  X  Y  (fig.  2.)  in  it,  to  converge  the  rays 
to  a  focus  on  the  object  A  B.  There  is  but  one  thing 
more  wanting  to  complete  the  solar  microscope,  which  I 
shall  leave  to  Caroline's  sagacity  to  discover. 

Caroline.  Our  microscope  has  a  small  mirror  attached 
to  it,  upon  a  moveable  joint,  which  can  be  so  adjusted  as 
to  receive  the  sun's  rays,  and  reflect  them  upon  the  ob- 
ject ;  if  a  similar  mirror  were  placed  to  reflect  light  upon 
the  lens,  would  it  not  be  a  means  of  illuminating  the  ob- 
ject more  perfectly  ? 

Mrs,  B,  You  are  quite  right.  P  Q,  (fig.  2.)  is  a 
small  mirror  placed  on  the  outside  of  the  window-shutter, 
which  receives  the  incident  rays  S  S,  and  reflects  them  on 
the  lens  X  Y.  Now  that  we  have  completed  the  appara- 
tus, let  us  examine  the  mites  on  this  piece  of  cheese, 
which  I  place  near  the  focus  of  the  lens. 

Caroline,  Oh !  how  much  more  distinct  the  image 
now  is,  and  how  wonderfully  magnified  ;  the  mites  on 
the  cheese  look  like  a  drove  of  pigs  scrambling  over  rocks. 

Emily,  I  never  saw  any  thing  so  curious.  Now  an 
immense  piece  of  cheese  has  fallen  :  one  would  imagine 
it  an  earthquake  :  some  of  the  poor  mites  must  have  been 
crushed  ;  how  fast  they  run, — they  absolutely  seem  tb 
gallop.  i 

But  this  microscope  can  be  used  only  for  transparent 
objects  ;  as  the  light  must  pass  through  them  to  form  the 
image  on  the  wall. 

Mrs,  B,  Very  minute  objects,  such  as  are  viewed  in 
a  microscope,  are  generally  transparent;  but  when  opaque 
objects  are  to  be  exhibited,  a  mirror  M  N  {^g,  3. )  is  used 
to  reflect  the  light  on  the  side  of  the  object  next  the  wall : 
the  image  is  then  formed  by  light  reflected  from  the  object, 
instead  of  being  transmitted  through  it. 

Emily,  Pray  is  not  a  magick  lantern  constructed  on 
the  same  principles  ?* 

^  The  magick  lantern  is  an  instrument  used  for  magnifying 
paintings  on  glass,  and  throwing  their  images  upon  a  white  screen 
in  a  darkened  chamber. 

1019.  What  does  fig.  2,  plate  XXIII.  represent .? 1020.  What 

is  the  use  of  the  mirror  in  the  solar  microscope  .^ 1021.     For 

what  objects  can  the  solar  microscope  be  used  .' 1022.     How 

can  opaque  objects  be  exhibited  .' 1023.     Which  figure  illus- 
trates this  ? 1024.     What  is  a  magich  lantern  ? 


opTicKs.  239 

Mrs,  B,  Yes ;  with  this  difference,  that  the  light  is 
supplied  by  a  lamp,  instead  of  the  sun. 

The  microscope  is  an  excellent  invention,  to  enable  us 
to  see  and  distinguish  objects,  which  are  too  small  to  be 
visible  to  the  naked  eye.  But  there  are  objects  which, 
though  not  really  small,  appear  so  to  us,  from  their  dis- 
tance ;  to  these  we  cannot  apply  the  same  remedy  ;  for 
when  a  house  is  so  far  distant,  as  to  be  seen  under  the  same 
angle  as  a  mite,  which  is  close  to  us,  the  effect  produced 
on  the  retina  is  the  same  :  the  angle  it  subtends  is  not 
large  enough  for  it  to  form  a  distinct  image  on  the  retina. 

Emily.  Since  it  is  impossible,  in  this  case,  to  approach 
the  object  to  the  eye,  cannot  we  by  means  of  a  lens  bring 
an  image  of  it  nearer  to  us  ? 

Mrs.  B.  Yes  ;  but  then  the  object  being  very  distant 
from  the  focus  of  the  lens,  the  image  would  be  too  small 
to  be  visible  to  the  naked  eye. 

Emily.  Then,  why  not  look  at  the  image  through  ano- 
ther lens,  which  will  act  as  a  microscope,  enable  us  to 
bring  the  image  close  to  the  eye,  and  thus  render  it  visi- 
ble ? 

Mrs.  B.  Very  well,  Emily  ;  I  congratulate  you  on 
having  invented  a  telescope.  In  figure  4,  the  lens  C  D, 
forms  an  image  E  F,  of  the  object  A  B  ;  and  the  lens  X 
Y,  serves  the  purpose  of  magnifying  that  image  ;  and  this 
is  all  that  is  required  in  a  common  refracting  telescope. 

Emily.  But  in  fig.  4,  the  image  is  not  inverted  on  the 
retina,  as  objects  usually  are  :  it  should  therefore  appear 
to  us  inverted ;  and  that  is  not  the  case  in  the  telescopes  1 
have  looked  through. 

Mrs.  B.  When  it  is  necessary  to  represent  the  image 
erect,  two  other  lenses  are  required  ;  by  which  means  a 
second  image  is  iormed,  the  reverse  of  the  first  and  conse- 
quently upright.  These  additional  glasses  are  used  to 
view  terrestrial  objects  ;  for  no  inconvenience  arises  from 
seeing  the  celestial  bodies  inverted, 

1025.  How  does  a  inagick  lantern  differ  from  a  solar  micro- 
scope?   1026.  What  is  the  reason  that  the  solar  microscope  may 

not  be  used  with  objects  at  a  great  distance  with  equal  effect .' 

1027.    What  does  Fig.  4,  plate  XXIII.  represent  ? 1028.    How 

would  you  explain  the  principle  of  the  common  refracting  tele- 
scope by  the  use  of  that  figure  ? 1029.     What  is  necessary  when 

the  image  of  an  object  is  to  be  exhibited  erect  ? 1030.  Why  are 

not  these  additional  glasses  used  in  viewing  celestial  objects  ? 


^0  OPTICKS. 

Emily,  The  difference  between  a  microscope  and  a 
telescope  seems  to  be  this  : — a  microscope  produces  a  mag- 
nified image,  because  the  object  is  nearest  the  lens  ;  and 
a  telescope  produces  a  diminished  image,  because  the  ob- 
jects farthest  from  the  lens. 

Mrs,  B,  Your  observation  applies  only  to  the  lens  C 
D,  or  object  glass,  which  serves  to  bring  an  image  of  the 
object  nearer  the  eye  ;  for  the  lens  X  Y,  or  eye-glass,  is  in 
fact  a  microscope,  as  its  purpose  is  to  magnify  the  image. 

When  a  very  great  magnifying  power  is  required,  tele- 
scopes are  constructed  with  concave  mirrors,  instead  of 
lenses.  Concave  mirrors,  you  know,  produce,  by  reflec- 
tion, an  effect  similar  to  that  of  convex  lenses  by  refrac- 
tion. In  reflecting  telescopes,  therefore,  mirrors  are  used 
in  order  to  bring  the  image  nearer  the  eye  ;  and  a  lens  or 
eye-glass  the  same  as  in  tlie  refracting  telescope  to  mag- 
nify the  image. 

The  advantage  of  the  reflecting  telescope,  is,  that  mir- 
rors whose  focus  is  six  feet,  will  magnify  as  much  as  lenses 
of  a  hundred  feet. 

Caroline,  But  I  thought  it  was  the  eye-glass  only  which 
magnified  the  image ;  and  that  the  other  lens  served  to 
bring  a  diminished  image  nearer  to  the  eye. 

Mrs,  B,  The  image  is  diminished  in  comparison  to 
the  object,  it  is  true  ;  but  it  is  magnified  if  you  compare 
it  to  the  dimensions  of  which  it  would  appear  without  the 
intervention  of  any  optical  instrument ;  and  this  magni- 
fying power  is  greater  in  reflecting  than  in  refracting 
telescopes. 

We  must  now  bring  our  observations  to  a  conclusion, 
for  I  have  communicated  to  you  the  ^hole  of  my  very 
limited  stock  of  knowledge  of  Natural  Philosophy.  If  it 
will  enable  you  to  make  further  progre^^^  in  that  science, 
my  wishes  will  be  satisfied ;  but  remember  that,  in  order 
that  the  study  of  nature  may  be  productive  of  happiness, 
it  must  lead  to  an  entire  confidence  in  the  wisdom  and 
goodness  of  its  bounteous  Author. 

1031.     What  part  of  the  telescope  exhibited  in  the  figure  may 

be  considered  as  a  simple  microscope  ? 1032.     When  a  very 

great  magnifying  power  is  required,  how  must  telescopes  be  con- 
structed ? 1033.     In  the  reflecting  telescopes  why  are  mirrors 

used  ? 1034.     How  great  is   the  advantage  of  the  reflecting 

telescope  ? 


A  DICTIONARY  OF  PHILOSOPHICAL  TERMS. 


ABERRATION,  in  astronomy,  an  appa- 
rent motion  of  the  heavenly  bodies,  pro- 
duced by  the  progressive  motion  of  light 
and  the  earth's  annual  motion. 

ACCELERATION,  in  mechanicks,  de 
notes  tlie  augmentation  or  increase  of  mo- 
tion in  accelerated  bodies. 
ACOUSTICKS  is  the  science  whicli  treat: 


pole,  66  1-2  degrees  from  the  equator  and 
parallel  to  it. 

AREOMETER,  an  instrument  by  which 
the  density  and  gravity  of  fluids  are  mea- 
sured. 

ARIES,  in  astronomy,  a  constellation  of 
fixed  stars,  drawn  on  the  globe  in  the  figure 
of  a  ram.   It  is  the  first  of  the  twelve  signs 


of  the  nature,  phenomena,  and  laws  of  the; of  the  zodiac  from  which  a  twelfth  part  of 
sense  of  sound.  It  extends  to  the  theory  of  jthe  ecliptick  takes  its  name.  It  consists  of 
musical  concord  and  harmony,  and  is,  there-,  sixty-six  stars. 

fore,  a  valuable  and  interesting  science.  |  ASCENSION,  in  astrononfi}',  the  rising 
AIR,  a  thin,  elastick  fluid,  surrounding  thelof  the  sun  or  star,  or  any  part  of  the  equi- 
globe  of  the  earth.  The  air,  together  withjnoctial  with  it,  above  the  horizon, 
the  clouds  and  vapours  that  float  in  it,  is  ASTERIODS,  a  nam-^  given  by  Dr.  Her- 
called  the  atmosphere.  The  height  to  whichjschel  to  the  new  planets,  Ceres,  Juno,  Pal- 
the  atmosphere  extends  has  never  been  as-  las,  and  Vesta,  lately  discovered. 


certained ;  but,  at  a  greater  height  than  45 
miles,  it  ceases  to  reflect  the  rays  of  light 
from  the  sun. 

AIR-PUMPS  are  machines  made  for  ex- 
hausting the  air  from  certain  glass  vessels, 
adapted  to  the  purpose  of  experiments  on 
air. 

ANGLE  is  the  inclination  of  two  linos 


ASTRONOMY  is  the  science  which 
teaches  the  motions  of  the  earth,  the  sun, 
moon,  planets,  comets,  and  stars,  and  ex- 
plains the  phenomena  occasioned  by  those 
motions. 

ATMOSPHERE,  or  atmospherickair,  the 
fluid  that  surrounds  our  earth.  Without 
this  fluid  no  animal  could  exist ;  vegetation 


meeting  one  another  in  a  point,  and  called] would  cease,  and  there  would  be  neither 
the  legs  of  the  angle.   Angles,  in  geometry, 'rain  nor  refreshing  dews  to  moisten  the 


are  called  rights  acute,  and  obtii?c.  A  right 
angle  contains  just  90  degree.:;  or  tlie  quar- 
ter part  of  a  circle.  Acute  nngles  contain 
Ies3,and  obtuse  angles  more  tlian  90  degrees. 

ANGLE  OF  INCIDENCE  is  that  which 
is  contained  between  the  lino  described  by 
the  incident  ray,  and  a  line  perpendicular 
to  the  surface  on  which  the  ray  ptrikes, 
raised  from  the  point  of  incidence. 

ANGLE  OF  REFLECTION  is  contain- 
ed between  the  line  described  by  the  re- 


face  of  the  ground  ;  and  though  the  sun  and 
stars  might  be  seen  as  bright  specks,  yet 
there  would  be  little  enjoyment  of  light, 
could  we  exist  v/ithout  it. 

ATTRACTION,  a  general  term,  used  to 
denote  the  power  or  principle  by  v.'hich  bo- 
dies mutually  tend  towards  each  other, 
without  regarding  the  cause  or  action  that 
niav  he  the  means  of  producing  the  eflTect. 

ATTRACTION  OF  COHESION  takes 
place  between  the  confrtituent  particles  of 


fleeted  rav;  and  a  lino  perpendicular  to  the  jthe  same  body.  By  this  principle  bodies 
reflecting  surface,  at  the  point  from  whif"h  preserve  their  forms  and  arc  prevented  from 
the  ray  is  reflected.  ifalling  to  piece*. 

ANGLE  OF  REFRACTION  is  thalj  ATTRACTION  OF  GRAVITATION, 
which  is  contained  between  tlie  line  descrili- or  gravity,  i.^  the  name  of  that  force  by 
ed  by  the  refracted  ray,  and  a  lino  perpendi-;  which  distant  bodies  tend  towards  one 
cular  to  the  refracting  surface  at  the  point  another. 


in  which  the  ray  passes  through  that  sur 

ANGLE  OF  VISION  is  th'at  which  is 
contained  between  lines  coming  from  oppo- 
site parts  of  an  object  and  meeting  in  the 


AXIS  of  a  planet  is  an  imaofinnry  line 
which  passes  through  its  centre,  and  on 
which  it  turns ;  and  it  is  this  motion  which 
produces  day  and  niglit.  V/ith  that  side  of 
the  planet  fHcin.^  the  snn  it  is  day ;  and 
with  the  opposite  si(?(j,  which  remains  in 
'ANTARCTICK CIRCLE,  in  astronomy.idarkness,  it  is  night, 
is  an  imaginary  Ivio  exteiiding  round  the'  AXIS  OP  MOTION,  in  mechanicks,  is  the 
south  pole,  661--i  degrees  from  tiie  equator, line  about  which  a  revolving  boc'y  moves, 
and  parallel  to  it.  j Philosophically  =peaking.  the  axis  of  mo- 

APHELION,  in  astronomy,  is  that  pointitinn  is  said  to  be  at  reat,  whilst  the  other 
in  any  planet's  orbit  in  which  the  orbit  is'parts  of  a  body  move  round  it;  and  the 
most  distant  from  the  sun.  Ifurth'"'-  any  part  af  a  body  is  from  the  axis 

AQ,U£OUS  HUiuOUR,  or  watery  hu-|of  motion,  the  greater  is  its  velocity, 
mour  of  the  eye ;  it  is  the  first  and  outermost, j  AXIS  OF  THE  EARTH  is  an  imagina- 
and  tint  which  is  less  dense  than  ciiheriry  line  conceived  to  pass  through  the  centre 
the  vitreous  or  crystalline.  It  i-,'  transpa-joV  it  from  one  pole  to  the  other,  u.'.ont  v/hich 
rent  and  colourless  like  water,  and  hlls  uplls  performnd  its  diurnal  rotation. 
:he  spaco  that  lies  between  tht  cornea  and'  AXIS,  in  opticks.  is  that  raj',  among  all 
ihe  crystalline  humour.  lotherg  that  arc  oont  to  the  eye^  which  falls  * 

ARCTICK  CIRCJ  iE,  in  astronomy,  is  an!  perpendicularly  upon  it,  and  which  consc- 
anaginary  line  expending  round  the  northi  quently  passes  through  the  centre  of  the  eye. 
'  21 


242 


A    DICTIONARY    OF 


AXIS  OF  A  GLASS,  OR  LENS,  is  a 

Tight  line  joining  the  middle  points  of  the 
two  opposite  surfacos  of  the  glass. 

BALANCE,  or  BALLANCE,  in  mecha- 
nicks,  one  of  the  simple  powers  which  serves 
to  find  out  the  equality  or  difference  of 
weight  in  heavy  bodies. 

BALLOON,  a  machine  used  in  naviga- 
tion through  the  air.  It  takes  its  name 
from  the  K)rm  of  the  machine,  the  word 
balloon  signifying  any  spherical  hollow  bo- 


that  point  about  which  all  th«  parts  of  a 
body  do,  in  any  situation,  balance  each 
other. 

CENTRE  OF  MOTION,  that  point 
which  remains  at  rest,  while  all  the  oth«r 
I  parts  of  a  bfulv  move  about  it. 

CENTRAL  FORCES,  the  poAvers  which 
cause  a  moving  body  to  tend  towards,  or 
recede  from,  the  centre  of  motion. 

CENTRIFUGAL  FORCE,  that  by 
which  all    bodies,  that  move  round  any 


dy,  of  whatever  matter  it  be  composed,  orjothor  body  in  a  curve,  endeavour  to  fly  off 
for  whatever  purposes  it  be  designed.  from  the  axis  of  their  motion  in  a  tangent. 

BAROMETER,  an  instrument  for  mea-  CENTRIPETAL  FORCE,  that  force  by 
suring  the  weight  or  pressure  of  the  at-  which  a  body  is  every  where  impelled,  or 
mosphere;  and  by  that  means  measuring  any  how  tends  towards  some  point  as  a 
heights  and  depths,  determining  variations  centre  ;    such   as  gravity,  or    that    force 


in  the  state  of  the  air,  and  foretelling  the 
changps  in  the  weather. 

BASE,  in  geometry,  the  lowest  side  of 
the  perimeter  of  a  figure.  Thus,  the  base 
of  a  triangle  may  be  said  of  any  of  its  sides, 
but  more  properly  of  the  lov/est,  or  that 
which  is  parallel  to  the  horizon.  In  rec- 
tangled  triangles,  the  base  is  properly  that 
side  opposite  to  the  right  angle. 

BASS,  in  musick,  that  part  of  a  concert 


hereby  bodies  tend  towards  the  centre  of 
the  earth  ;  magnetical  attraction,  whereby 
the  loadstone  draws  iron  ;  and  that  force, 
whatever  it  be,  whereby  the  planets  are 
continually  drawn  back  from  right-lined 
motions,  and  made  to  move  in  curves, 

CHROMATICKS  is  that  part  of  opticks 
which  explains  the  several  properties  of 
tlie  colours  of  light  and  of  natural  bodies. 
CIRCLE,  in  geometry,  a  plane  figure 
which  is  most  heard,  which  consists  of  thejcomprehended  by  a  single  curve  line,  called 
gravest  and  deepest  sounds,  and  which  isjits  circumference,  to  wnich  right  lines  or 
played  on  the  largest  pipes  or  strings  of  the  iradii,  drawn  from  a  point  in  the  middle, 
instrument.  called  the  centre,  are  equal  to  each  other. 

BODY,  in  physicks,  an  extended  solid  sub-  CIRCUMFERENCE,  in  a  general  sense, 
stance,of  itself  utterly  passive  and  inactive, jdenotes  the  line  or  lines  bounding  a  plane 
indifferent  either  to  motion  or  rest  ;  but  ca-jfigure.  However,  it  is  generally  used  in  a 
pable  of  any  sort  of  motion,  and  of  all  figures  ;more  limited  sense,  for  the  curve  line  which 
and  forms.     Body,  or  substance,  which   isjbounds  a  circle,  and  otherwise  called  a  pe- 


the  same  thing,  is  usually  denoted  by  the 
-  general  term  matter. 

BREADTH,  in  geometry,  one  of  the 
three  dimensions  of  bodies,  which,  multi- 
plied into  their  length,  constitutes  a  sur- 
face. 

BUBBLE,  in  philosophy,  small  drops  or 
vesicles  of  any  fluid  filled  with  air,  and 
either  formed  on  its  surface,  by  an  addition 


riphery ;  the  boundary  of  a  right  lined 
figure  being  expressed  by  the  term  perime- 
ter. 

CLOUDS  are  a  collection  of  misty  va- 
pours suspended  in  the  air.  Their  various 
colours  and  appearances  are  owing  to  their 
particular  situation  in  regard  to  the  sun, 
to  the  different  reflection  of  the  sun's  rays, 
and  to  the  effects  produced  on  them  by 

of  more  of  the  fluid,  or  in  its  substance,  by!  wind. 

an  intestine  motion  of  its  component  parts.  |     COHESION,  one  of  the  species  of  attrac- 
BURNING-GLASS,  a  convex  or  concavejtion,  denoting  that  force  by  which  the  parts 

glass,  commonly   spherical,  which,  beingiof  bodies  stick  together. 

exposed  directly  to  the  sun,  collects  all  the      COLOUR  means  that  property  of  bodies 

rays  falling  thereon  into  a  very  small  space. 


called  the  focus,  where  wood,  or  any  other 
combustible  substance,  being  put,  will  be 
set  on  fire. 

CAMER A-OBSCURA,  in  opticks,  a  ma- 
chine representing  an  artificial  eye.    It  is 


which  affects  the  sight  only  ;  thus  the  grass 
in  the  fields  has  a  green  colour,  blood  has  a 
red  colour,  the  sky  generally  appears  of  a 
blue  colour,  and  thus  of  others  that  might 
be  named.  The  variety  of  colours,  as  they 
are  presented  to  us  by  the  substances  that 


made  by  placing  a  convex  glass  in  a  hole  .surround  us,  is  immense,  and  from  them 
of  a  window  shutter,  and  if  no  light  enters  arises  the  admirable  beauty  of  the  works 
theroombutthroujh  the  glass,  the  pictures  of  nature  in  the  animal,  in  the  vegetable, 
of  all  objects  on  the  outside  may  be  distinct-  and  in  the  mineral  kingdom,  or,  more  pro- 
ly  seen  in  an  )nverted  position,  on  any  perly  speaking,  in  the  universe, 
white  surface  placed  at  the  focus  of  the  COLURES,  in  astronomy  and  geogra- 
lens.  phy»  ^  wo  great  circles,  supposed  to  inter- 

CAPILLARY  TUBES,  in  physicks,  little  sect  each  other  at  right  angles  in  the  poles 
pij)e9,  whose  canals  are  extremely  narrow, 'of  the  world,  and  to  pass  through  the  sol- 
used  for  experiments  in  illustrating  cohe-'stitial  and  equinoctial  points  of  the  eclip- 
sive  attraction.  jtick. 

CAPPTCORN,  in  astronomy,  one  of  thej  COMETS  are  opaque  and  solid  bodieg. 
twelve  signs  of  the  zodiack,  represented  onjAcomet,  atagiven  distance  from  the  earth, 
globes  in  the  form  of  a  2 oat.  jshines  much  brighter  when  it  is  on  the 

CENTREOFGRAVlTYjinmechanickSj'samesideof  the  earth  with  the  sun  than 


PHILOSOPHICAL  TEEMS- 


24$ 


when  it  is  on  the  contrary  side;  from  ing  to  make  that  refraction  of  the  rays  of 
whence  it  appears  that  it  owes  its  bright-^ light,  necessary  to  make  them  meet  in  the 
ness  to  the  sun.  [retina,  and  form  an  image  thereon,  where- 

COMPLEMENT,  in  astronomy,  the  dis-jby  vision  may  be  performed, 
tance  of  a  star  from  the  zenith  ;  or  the  arch]     CYLINDER,  in  geometry,  a  solid  body, 
comprehended  between  the  place    of  the  supposed  to  be  generated  by  the  rotation 
star  above  the  horizon  and  the  zenith.         j  of  a  parallelogram. 

COMPRESSION,  the  act  of  pressing  or]  DAY.  In  common  language,  the  day  is 
squeezing  some  matter,  so  as  to  set  its  the  interval  of  time  which  elapses  from  the 
parts  nearer  to  each  other,  and  make  iti rising  to  the  setting  of  the  sun.  The  astro- 
possess  less  space.  nomical  day  embraces  the  whole  interval 

CONCAVE,    an    appellation    used     in  which  passes  during  a  complete  revolution 
speaking  of  the  inner  surface  of  hollow  bo-  of  the  sun. 
dies,  but  more  especially  of  spherical  ones.  I     DECLINATION,  in  astronomy,  the  dis- 

CONCORD,in  musick,  the  relation  of  two|  tance  of  any  celestial  object  from  the  equi- 
sounds  that  are  always  agreeable  to  the  noctial,  either  northward  or  southward.  It 
ear,  whether  applied  in  succession  or  con-, is  either  true  or  apparent,  according  as  the 
sonance.  jreal  or  apparent  place  of  the  object  is  con- 

CONDENSER,  a  pneumatick  engine  or,sidered. 
syringe,  whereby  an  uncommon  quantity!  DEGREE,  in  geometry,  a  division  of  a 
of  air  may  be  crowded  into  a  given  space  ;[ circle,  including  a  three  hundred  and  six- 
80  that  sometimes  ten  times  as  much  air  as, tieth  part  of  its  circumference.  Every 
there  is  at  the  same  time  in  the  same  space,  circle  is  supposed  to  be  divided  into  three 
without  the  engine,  may  be  thrown  in  by  hundred  and  sixty  parts,  called  degrees, 
means  of  it,  and  its  egress  prevented  by  and  each  degree  divided  into  sixty  other 
valves  properly  disposed.  I  parts  called  minutes  ;    and  each  of  these 

CONDUCTORS,  in  electricity,  are  long  minutes  is  again  divided  into  sixty  seconds, 
metal  rods,  whose  points  are  raised  above]  DENSITY  denotes  the  degree  of  close- 
the  buildings  to  which  the  conductors  arc  ness  and  compactness  of  the  particles  of  a 
affixed,  for  the  purpose  of  attracting  or  re-:  body  ;  and  is  that  property  directly  oppo- 
ceiving  the  electrick  fluid,  and  of  coaduct-isite  to  rarity. 

ing  it  into  the  earth,  or  into  water,  thereby  |  DEPRESSION  OF  THE  POLE.  When 
to  prevent  such  buildings  from  being  struck^  person  sails  or  travels  towards  the  equa- 
by  lightning.  I  tor,  he  is  said  to  depress  the  pole,  because 

CONE,  in  geometry,  a  solid  figure,  hav-  as  many  degrees  as   he  approaches  nearer 


ing  a  circle  for  its  base,  a^id  its  top  termi- 
nated in  a  point  or  vc'^ex. 

CONJUNCTION^  in  astronomy,  is  the 
meeting  of  twost-^rs  or  planets  in  the  same 
degree  of  the  r-odiack. 

CONSTFi^LATION,  in  astronomy, 
system  ©''"several  stars  that  are  seen  in  the 
heaver^  near  to  one  another.  Astronomers 
not  only  mark  out  the  stars,  but  they  dis 
trioute  them  into  asterisms,  or  constella- 
tions, allowing  several  stars  to  makeup 
one  constellation  ; — and  for  the  better  dis- 
tinguishing and  observing  them,  they  re- 
duce the  constellations  to  the  forms  of  ob- 
jects with  which  we  are  well  acquainted. 

CONVERGING,  or  convergent  lines,  in 
geometry,  are  such  as  continually  approach 
nearer  one  another  ;  or  whose  distance  be- 
comes still  less  and  less. 

CONVERGING  RAYS,  in  opticks,  are 
those  rays,  that,  issuing  from  diverse  points 
of  an  object,  incline  towards  one  another, 
till,  at  last,  they  meet  and  cross,  and  then 
become  diverging  rays. 

CONVEX,  an  appellation  given  to  the 


the  equator,  so  many  degrees  will  the  pole 
be  nearer  the  horizon.  The  phenomenon 
arises  from  the  spherical  figure  of  the  earth. 

DIAGONAL,  in  geometry,  a  right  line 
drawn  across  a  quadrilateral  figure,  from 
one  angle  to  another,  by  some  called  the 
diameter  of  the  figure. 

DIAMETER,  in  geometry,  a  right  line 
passing  through  the  centre  of  a  circle,  and 
terminated  at  each  side  by  the  circumfe- 
rence thereof 

DIGIT,  in  astronomy,  the  twelfth  part 
of  the  diameter  of  the  sun  or  moon,  is  used 
to  express  the  quantity  of  an  eclipse.  Thus 
an  eclipse  is  said  to  be  six  digits,  when  six 
of  these  parts  are  hid. 

DIMENSION,  in  geometry,  is  either 
breadth,  length,  or  thickness  ;  hence  a  line 
has  one  dimension,  viz.  length  ;  a  superfi- 
cies, two,  viz.  length  and  breadth ;  and  a 
body,  or  solid,  has  three,  to  wit,  length, 
breadth,  and  thickness. 
DIRECTION,  inmechanicks,  signifies  the 
line  or  path  of  a  body's  motion,  along  which 
it  endeavours  to  proceed,  according  to  the 


exteriour  surface  of  gibbous  or  globular  bo-i force  impressed  upon  it. 
dies,  in  opposition  to  the  hollow  inner  sur-  DISK,  in  astronomy,  the  body  and  face 
faceof  such  bodies,  which  is  called  concave,  of  the  sun  and  moon,  such  as  it  appears  to 
Thus  we  say  a  convex  lens,  a  convex  mir-  us  on  the  earth,  or  the  body  or  face  of  the 
ror,  and  convex  superficies.  earth,  such  as  it  appears  to  a  spectator  in 

CORNEA,  the  second  coat  of  the  eye,  so  the  moon.  The  disk  in  eclipses  is  supposed 
called  from  its  substance,  which  resembles, to  be  divided  into  twelve  equal  parts, 
the  horn  of  a  lantern.  DISCORD,  in  musick,  a  dissonant  and  un- 

CRYSTALLINE  HUMOUR,a  thick  com-|harmonious  combination  of  sounds,  so  call- 
pact  humour,  in  form  of  a  fljittish  convex  ed  in  opposition  to  concord, 
lens,  situated  in  the  middle  of  the  eye,  se.rv-j     DIVERGENT  RA,YS,  in    opticks,  are 


244 


A   DICTIONARY    OP 


those,  which,  going  from  a  point  of  the  vi- 
sible object,  are  dispersed,  and  continually 
depart  one  from  another,  in  proportion  as 
they  are  removed  from  the  object ;  in  which 
•ensc  it  is  opposed  to  convergent. 

DIVISIBILITY,  that  property  by  which 
the  particles  of  matter  in  all  bodies  are  ca- 
pable of  a  separation,  or  disunion  from 
each  other. 

DIURNAL,  in  astronomy,  something  re- 
lating to  the  day,  in  opposition  to  noctur- 
nal, which  regards  the  night.  The  diurnal 
motion  of  a  planet,  is  so  many  degrees  and 
minutes  as  any  planet  moves  in  twenty- 
four  hours.  Hence  the  motion  of  the  earth 
about  its  axis  is  called  its  diurnal  motion. 

DROPS,  in  meteorology,  small  spherical 
bodies,  into  which  the  particles  of  fluids 
spontaneously  form  themselves,  when  let 
fall  from  any  height. 

DUCT  denotes  any  tube  or  canal. 

DUCTILITY,  in  physicks,  a  property  of 
certain  bodies,  whereby  they  are  capable  of 
being  expanded,  or  stretched  forth  by  means 
of  a  hammer  or  press. 

DYNAMICKS.  Tliis  branch  of  mecha- 
nicks  relates  to  the  action  offerees  that  give 
motion  to  solid  bodies ;  which  forces  are  cal- 
culated, both  by  their  active  powers,  and 
by  the  proportion  of  time  in  which  those 
powers  become  efficient. 

EARTH,  the  vast  mass  or  planet  which 
we  inhabit.  The  ancients  supposed  the 
earth  flat  or  cylindrical ;  but  from  the  ge- 
neral appearance  of  the  planetary  system, 
from  the  circular  shadow  of  the  earth  in 
eclipses  of  the  moon,  and  from  the  fact  that 
the  earth  has  been  circumnavigated,  it  is 
concluded  by  the  moderns,  that  it  is  sphe- 
rical. 

EARTHaUAKE  is  a  sudden  motion  of 
the  earth,  occasioned,  it  is  supposed,  either 
by  the  discharge  of  some  electrical  power, 
or  by  large  quantities  of  inflammable  air. 
which,  on  being  rarefied  by  internal  fires, 
forces  its  way  through  the  parts  that  sur- 
round it. 

EAST,  one  of  the  four  cardinal  points  of 
the  world  ;  being  that  point  of  the  horizon, 
■where  the  sun  is  seen  to  rise  when  in  the 
equinoctial. 

ECCEN  TRICK,  in  geometry,  a  term  ap- 
plied to  circles  and  spheres  which  have  not 
the  same  centre,  and  consequently  are  not 
parallel,  in  opposition  to  concentrick,  where 
they  are  parallel,  having  one  common  cen- 
tre. 

ECCENTRICITY,  in  astronomy,  is  the 
distance  of  the  centre  of  the  orbit  of  a  pla- 
net from  the  centre  of  the  sun,  that  is,  the 
distance  between  the  centre  of  the  ellipsis 
and  the  focus. 

ECHO,  a  sound  reverberated  or  reflected 
to  the  ear  from  some  solid  body. 

ECLIPSE,  the  deprivation  of  the  light  of 
the  sun,  or  of  some  heavenly  body,  by  the 
interposition  of  another  neavenly  body  be- 
tween it  and  our  sight. 

ECLIPTICK,  in  astronomy,  a  great  circle 
of  the  sphere,  supi)0sed  to  be  drawn  through 
the  middle  of  thv  zodiack ;  or  it  is  that  path 


among  the  fixed  stars,  that  the  earth  ap- 
pears to  describe,  to  an  eye  placed  in  the 
sun. 

ELASTICITY,  that  disposition  in  bo- 
dies by  which  they  endeavour  to  restore 
themselves  to  the  posture  from  whence 
they  were  displaced  by  an  external  force. 

ELECTRICITY  is  an  invisible,  subtile 
fluid,  that  appears  to  pervade  all  nature^ 
and  among  other  interesting  phenomena,  is 
the  cause  of  thunder  and  lightning.  Elec- 
tricity is  of  two  kinds — positive  and  nega- 
tive. The  positive  is  that  state  of  a  body 
which  contains  more  than  its  due  propor- 
tion ;  and  the  negative  is  that  state  of  a 
body  which  contains  less  than  its  due  pro- 
portion. When  two  bodies,  one  charged 
with  positive  electricity  and  the  other  with 
negative,  come  in  contact  with  each  other, 
so  much  passes  from  the  former  to  the  lat- 
ter, as  to  produce  an  equilibrium — it  passes 
thus  with  a  flash  and  an  explosion.  Thus 
two  clouds,  charged  in  the  above  manner, 
coming  together,  or  one  cloud  coming  in 
contact  with  the  earth,  thunder  and  light- 
ning are  produced. 

ELLIPSIS,  in  geometry,  a  curve  line  re- 
turning into  itself,  and  produced  from  the 
section  of  a  cone  by  a  plane  cutting  both  its 
sides,  but  not  parallel  to  the  base. 

EMERSION,  in  astronomy,  is  when  any 
planet  that  is  eclipsed  begins  to  emerge  or 
get  out  of  the  shadow  of  the  eclipsing  body. 
It  is  also  used  when  a  star,  before  nidden 
by  the  sun,  as  being  too  near  him,  begins  to 
re-appear  or  etnerge  out  of  his  rays. 

E(iUATOR  is  aR  imaginary  circle  equal- 
ly distant  from  the  pUes,  and  dividing  the 
earth  into  two  equal  pars,  one  being  called 
the  Northern  hemisphere,  a»{i  the  other  the 
Southern  hemisphere. 
,  EaUINOCTI  AL,  in  astrononrj,  a  great 
circle  of  the  celestial  globe,  whose  poles  are 
the  poles  of  the  world.  It  is  so  called,  be- 
cause, whenever  the  sun  comes  to  this  cir- 
cle, the  days  and  nights  are  equal  all  over 
the  globe  ;  being  the  same  with  that  which 
the  sun  seems  to  describe  at  the  time  of  the 
two  equinoxes  of  spring  and  autumn. 

EQ,UINOX,  the  time  when  the  sun  en- 
ters either  of  the  equinoctial  points,  where 
the  ecliptick  intersects  the  equinoctiah 

EXHALATION,  a  general  term  for  all 
the  effluvia  or  streams  raised  from  the  sur- 
face of  the  earth  in  form  of  vapour.  Some 
distinguish  exhalations  from  vapours,  ex- 
pressing by  the  former  all  steams  emitted 
from  solid  bodies,  and  by  the  latter,  the 
steams  raised  from  water  and  other  fluids. 

EXPANSION,  the  enlargement  or  in- 
crease of  bulk  in  bodies,  chiefly  by  means 
of  heat. 

EXPLOSION,  a  sudden  and  violent  ex- 
pansion of  an  aerial  or  other  elastick  fluid, 
by  which  it  instantly  throws  off"  any  obsta- 
cle that  happens  to  be  in  the  way,  some- 
times with  incredible  force,  and  in  such  a 
manner  as  to  produce  the  most  astonishing 
effects. 

EXTENSION,  in  philosophy,  one  of  the 
common  and  essential  properties  of  body, 


Philosophical  terms. 


245 


ofr  that  by  which  it  possesses  or  takes  up 
some  part  of  universal  space,  which  is  call- 
ed the  place  of  a  body. 

FIGURE,  in  physicks,  expresses  the  sur- 
face, or  terminating  extremities  of  any  bo- 
dy ;  and,  considered  as  a  property  of  body 
affecting  our  senses,  is  defined  a  quality 
which  may  be  perceived  by  two  of  the 
outward  senses.  Thus  a  table  is  known 
to  be  square  by  the  sight  and  by  the 
touch. 

FLUID,  in  physiology,  an  appellation 
given  to  all  bodies  whose  particles  easily 
yield  to  the  least  partial  pressure  or  force 


FOCUS,  in  geometry  and  conick  sections, 
is  applied  to  certain  points  in  the  parabola, 
ellipsis,  and  hyperbola,  where  the  rays  re- 
flected from  all  parts  of  these  curves  con- 
cur and  meet. 

FOGS  are  clouds  which  float  on  the  sur- 
face of  the  earth,  and  clouds  are  fogs  in  the 
higher  regions  of  the  atmosphere  ;  from 
many  places  they  may  be  seen  floating  in 
the  vallies,  and  often  in  the  vallies  they 
may  be  seen  creeping  along  the  sides  of  the 
mountains. 

FORCE,  in  mechanicks,  denotes  the  cause 
of  the  change  in  the  state  of  a  body,  when, 
being  at  rest,  it  begins  to  move,  or  has  a 
motion  which  is  either  not  uniform  or  not 
direct.  Mechanical  forces  may  be  reduced 
to  two  sorts,  one  of  a  body  at  rest,  the  other 
of  a  bodj'  in  motion. 

FORCING-PUMP,  in  mechanicks,  a  kind 
of  pump  in  which  there  is  a  forcer  or  piston 
without  a  valve. 

FOUNTAIN,  in  philosophy,  a  spring  or 
source  of  water  rising  out  of  the  earth. 

FRICTION,  in  mechanicks,  the  rubbing 
of  the  parts  of  engines  and  machines  against 
each  other,  by  which  means  a  great  part 
of  their  effect  is  destroyed. 

FRIGID  ZONES,  the  spaces  on  the 
earth's  surface  between  the  polar  circles 
and  the  poles. 

FULCRUM,  in  mechanicks,  the  press  or 
support,  by  which  a  lever  is  sustained. 

GALAXY,    in    astronomy,  a   very 


into  drops,  and  are  frozen  while  they  are 
falling.  They  assume  various  figures,  be- 
ing sometimes  round,  at  other  times  pyra- 
midal, cuniated,  angular,  thin  and  flat,  and 
sometimes  stellated  with  six  radii  like  the 
small  crystals  of  snow. 

HALO,  in  physiology,  a  meteor  in  the 
form  of  a  luminous  ring  or  circle,  of  vari- 
ous colours,  appearing  round  the  bodies  of 
the  sun,  moon,  or  stars. 

HARDNESS,  in  physiology,  is  the  resist- 
ance opposed  by  a  body  to  the  separation 
of  its  particles.  This  property  depends  on 
the  force  of  cohesion  ;  and  a  body  is  con- 
sidered more  hard  in  proportion  as  it  pre- 
sents a  greater  resistance  to  the  force  which 
may  be  applied  in  order  to  separate  its 
parts. 

HARMONY,  in  musick,  the  agreeable 
result,  or  union,  of  several  musical  sounds, 
heard  at  one  and  the  same  time,  or  the  mix- 
ture of  divers  sounds,  which  together  have 
an  effect  agreeable  to  the  ear.  As  a  con- 
tinued succession  of  musical  sounds  pro- 
duces melody,  so  does  a  continued  combi- 
nation of  these  produce  harmony. 

H ARxMONY  OF  THE  SPHERES,  a  sort 
of  musick  much  talked  of  by  many  of  the 
ancient  philosophers,  supposed  to  be  pro- 
duced by  the  sweetly  tuned  motions  of  the 
stars  and  planets.  This  harmony  they  at- 
tributed to  the  various  proportionate  im- 
pressions of  the  heavenly  globes  upon  one 
another,  acting  at  proper  intervals. 

HEIGHT,  ir  geometry,  is  a  perpendicu- 
lar let  fall  fr'jm  the  vertex,  or  top,  of  any 
right-lined  ^Jgure,  upon  the  base  or  side 
subtending  It.  It  is  likewise  the  perpendi- 
cular hein«t  of  any  object  above  the  hori- 
zon. 

HEMISPHERE,  the  half  of  a  globe  or 
spliere,  when  it  is  supposed  to  be  cut 
throi'gh  its  centre  in  the  plane  of  one  of  its 
great  circles. 

KORIZON,  in  astronomy  and  geography, 
thit  great  circle  which  divides  the  heavens 
aid  the  earth  into  two  equal  parts  or  he- 
riispheres,  distinguishing  the  upper  from 
he  lower.     The  horizon  is  either  sensible 


markable  appearance,  sometimes  double,  or  rational — t he  sensible  horizon  is  that  cir- 


but  for  the  most  part  single,  surrounding 
the  whole  concave  of  the  heavens,  called 
the  galaxy  or  milky  way. 

GIBBOUS,  in  astronomy,  a  term  used  in 
reference  to  the  enlightened  parts  of  the 
moon,  whilst  she  is  moving  from  her  first 
quarter  to  the  full,  and  from  the  full  to  th' 
last  quarter. 

GLOBE,  a  round  or  spherical  body,  pOie 
usually  called  a  sphere,  bounded  h/  one 
uniform  convex  surface,  every  pc^nt  of 
which  is  equally  distant  from  a  po't^t  with- 
in called  th«3  centre. 

GRAVITY,  a  term  used  >y  physical 
writers  to  denote  the  cause  by  which  all 
bodies  move  towards  eac-'i  other,  unless 
prevented  by  some  other  ferce  or  obstacle. 

GREEN,  one  of  the  original  colours  ex- 
cited by  the  rays  of  light. 

HAIL,  a  compact  mass  of  frozen  water, 
consisting  of  such  vapours  as  are  united 
21  * 


cle,  which  being  discovered  by  our  senses, 
limits  oni  prospect. 

HORIZONTAL,  something  relating  to 
the  "lorizon  ;  or  that  is  taken  in,  or  on  a 
le- el  with  the  horizon.  Thus,  we  say,  a 
lorizontal  plane. 

HURRICANES  are  violent  storms,  fre- 
quent in  South  America  and  the  West  In- 
die3,^and  other  hot  countries,  in  which  the 
wind  changes  in  a  short  time  to  every  point 
of  the  compass,  and  blows  with  a  violence 
which  scarcely  any  thing  can  resist. 

HYADES,  in  astronomy,  seven  stars  in 
the  bull's  head,  famous  among  the  poets 
for  the  bringing  of  rain. 

HYDRA,  in  astronomy,  a  southern  con- 
stellation, imagined  to  represent  a  water 
serpent. 

HYDRAULICKS  teach  us  to  ascertain 
the  velocity  and  impetus  of  fluids  when  in 
motion,  and  serves  as  the  basis  for  comput- 


246  A    DICTIONARY    OF 

ing  the  powers  of  various  machinery  acted|the  place,  whose  latitude  is  spoken  of,  is  om 
upon  by  running  water.  jthis  or  that  side  of  the  equator. 

HYDROMETKll,  an  instrument  to  mca-l  LATITUDE,  in  astronomy,  the  distance 
sure  the  extent  and  specificit  gravity  of  lof  a  star  or  planet  from  the  ecliptick,  in 
fluids.  degrees,  minutes,  and  seconds,  measured  on 

HYDROSTATICAL  BALANCE,  a  kindia  circle  of  latitude  drawn  through  that  star 
of  balance  contrived  for  the  easy  and  exactjor  planet,  being  either  north  or  south,  as 
ndingthe  specitick  gravities  of  bodies  both  j  the  object  is  situated  either  on  the  north  or 
liquid  and  solid.  isouth  side  of  the  ecliptick. 

HYDROSTATICAL  PARADOX  is  thisj  LEE,  an  epithet  to  distinguish  that  half 
— thatany  quantity  of  riuid,  however  smalljiof  the  horizon  to  which  the  wind  is  direct- 
may  be  made  to  balance,  or  counterpoise  led  from  the  other  part  where  it  arises, 
any  quantity,  however  large.  which  latter  is  accordingly  called  towind- 

HYDROriTATICKS  treat  of  the  nature,  ward, 
gravity,  }»res3ure,  and  motion  of  fluids  inj  LENS  properly  signifies  a  small  round- 
general,  and  of  the  methods  of  weighingiish  glass,  of  the  figure  of  a  lentil,  but  is  ex- 
solids  in  them.  jtended  to  any  optick  glass,  not  very  thick, 
IMAGE,  inopticks,  is  the  appearance  of  i  which  either  collects  the  rays  of  light  into 
an  object  made  either  by  reflection  or  re-'a  point,  in  their  passage  through  it,  or  dis- 
fraction.  In  all  plane  mirrors,  the  image iporses  them  further  apart,  according  to  the 
is  of  the  same  magnitude  as  the  object,  and  laws  of  refraction. 

it  appears  as  far  behind  the  mirror  as  the!  LEO,  in  astronomy,  one  of  tlie  twelve 
object  is  before  it.  In  concave  mirrors  the  signs  of  the  zodiack,  the  fifth  in  order, 
object  appears  larger,  and  in  those  which  I  LEVEL,  an  instrument  constructed  for 
are  convex,  it  appears  less  than  the  object,  j  the  purpose  of  ascertaining  the  exact  level 
IMMERSION,  in  astronomy,  is  wlien  alof  any  fluid,  building,  or  any  other  object, 
star  or  planet  is  so  near  the  sun,  with  re-JLeveis  are  of  two  kinds — the  horizontal, 
gard  to  our  observations,  that  we  cannot 'and  the  perpendicular, 
see  it ;  being  as  it  v.ere  enveloped  or  hidden!  LEVER,  in  mechanicks,  an  inflexible 
in  the  rays  of  that  luminary.  It  also  de-jright  line,  rod,  or  beam,  supported  in  a  sin- 
notes  the  beginning  of  an  eclipse  of  thesun  gle  point  on  a  fulcrum  or  prop,  and  used 
or  moon,  when  either  of  those  bodies  begins  for  the  raising  of  weights;  being  either 
to  be  darkened  by  the  siudowofthe  oiher.jvoid  of  weight  itself,  or  at  least  having 
IMPENETRABILITY,  in  philosophy,isuch  a  weight  as  may  be  commodiously 
that  property  of  a  body  whs^reby  it  cannot  counterbalanced. 

be  pierced  by  another;  thus, x  body,  whichi  LIBRA,  the  balance,  in  astronomy,  onfr 
80  fills  a  space  as  to  exclude  ^J1  others,  isjof  the  twelve  signs  of  the  zodiack,  the  sixth 
said  to  be  impenetrable.  jin  order  ;  so  called,  because  when  the  sun 

INCIDENCE,  in    mechanicks,   denoteslenters  it,  the  days  and  nights  are  equal,  as 
the  direction  ia  which,  one  body  s^.rikes  on, if  weighed  in  a  balance, 
another.  ^  LIBRATION,  in  astronomy,  an  appa- 

INCLINATION,  is  a  word  frecf^ently'rent  inequality  of  the  moon's  motion, 
used  by  mathematicians,  and  signitie?  the! whereby  she  seems  to  librate  about  her 
mutual  approach,  tendency,  or  leaning  of  axis,  sometimes  from  the  east  to  the  west, 
two  lines,  or  planes,  towards  each  other,  so  and  now  and  then  from  the  west  to  the 
as  to  make  an  angle.  '     Jeast ;  so  that  the  parts  in  the  western  limb 

INCLINED-PLANE,  in  mechanicks,  is  ior  margin  of  tlie  moon  sometimes  recede 
merely  a  line  or  plane  that  makes  an  angk  from  the  centre  of  the  disk,  and  sometimes, 
with  the  horizon.  It  is  frequently  u.sed  toVnove  towards  it,  by  which  means  they  be- 
move  Aveights  from  one  level  to  anotlier.      ^tome  alternately  visible  and  invisible  ta 

INERTIA,  or  inactivity,  isihat  proi>er-!the  inhabitants  of  the  earth, 
ty  of  matter  by  which  It  would  aHays  con- 1    LIGHT  is  that  principle,  or  thing,  by 
tinue  in  the  same  state  of  rest,  or  ^»f  mo-  which  objects  are  made  perceptible  to  our 
tion,  ill  which  it  was  put,  unless  cha»^ed  sense  of  seeing  ;  or  the  sensation  occasion- 
by  some  external  force.  led  in  the  mind  by  the  view  of  luminous  ob- 

INTEGRAL,  or  integrant,  appeilationsjgct?; 
given  to  parts  of  bodies  which  are  of  a  si-,    LIGHTNING,  an  electrical  explosion, 
milar  nature  with  the  whole.  Thus,  filings!    tjjve,  in  geometry,  a  quantity  extended 
of  iron  have  the  same  nature  and  properties  in   tyigth   only,  without  any  breadth  or 
as  bars  of  iron.  ithick»ess. 

INTENSITY,  in  physieks,  is  the  degree  LiaviD,  a  fluid  not  sensibly  elasticfc, 
or  rate  of  power  or  energy  of  any  quality,  the  part»of  which  move  on  each  other,  and 
as  of  heat  and  cold.  lyield  to  th>  smallest  impression. 

JUPITER,  in  astronomy,  one  of  the  pri-!  L0NGIT\IDE,  in  geography,  is  an  arch 
mary  planets  remarkable  for  its  great  of  the  equate.-,  intercepted  between  the 
brightness.  first  meridian  passing  through  the  propos- 

LATITUDE,  the  distance  of  a  place  ed  place  ;  which  n  always  equal  to  the  an- 
froni  the  equator,  or  an  arc  of  the  meridi-  gle  at  the  pole,  fonrved  by  the  first  meridian 
an  intercepted  between  the  zeiith  of  the  and  the  meridian  of  the  place, 
place  and  the  equator.    Hence  latitude  is]     LOOKING-GLASSES  are  nothing  but 
either  northern  tir  southernj  according  as  plain  mirrors  of  glass,  which,  being  impei* 


PHILOSOPHICAL  Terms. 


247 


vious  to  the  light,  reflect  the  images  of 
things  placed  before  them. 

LUJ\AR,  something  belonging  to  the 
moon ;  thus  we  say,  lunar  month,  lunar 
year,  lunar  dial,  or  lunar  eclipse. 

LUNATION,  the  time  or  period  from 
one  new  moon  to  another — it  is  called  the 
synodical  month. 

MAGICK  LANTERN  is  an  instrument 
used  for  magnifying  paintings  on  glass,  and 
throwing  their  images  upon  a  white  screen 
in  a  darkened  room. 

MAGNETLSM  explains  the  properties 
of  the  loadstone,  or  natural  magnet,  which 
is  a  dark  coloured  and  hard  mineral  body, 
and  is  found  to  be  an  ore  of  iron,  being  ge- 
nerally found  in  iron  mines. 

MAGNITUDE,  whatever  is  made  up  of 
parts  locally  extended,  or  that  has  s  everal 
dimensions ;  as  a  line,  a  surface,  or  a  solid. 

MAN03IETER,  an  instrument  to  show 
or  measure  the  alterations  in  the  rarity  or 
density  of  the  air. 

MARS,  in  astronomy,  the  planet  that  re- 
volves next  beyond  the  earth  in  our  system, 
is  of  a  red  fiery  colour,  and  always  gives  a 
much  duller  light  than  Venus,  though  some- 
times he  equals  her  in  size. 

MATHEMATICKS  originally  signified 
any  discipline  or  learning  ;  but  at  present, 
denotes  that  science  which  teaches,  or  con- 
templates whatever  is  capable  of  being 
numbered  or  measured,  in  so  far  as  it  is 
computable  or  measurable;  and  according- 
ly is  subdivided  into  arithmetick,  which 
has  numbers  for  its  object,  and  geometry, 
which  treats  of  magnitudes. 

MATTER  is  the  general  name  of  every 
substance,  that  has  length,  breadth,  and 
thickness. 

MECHANICKS,  is  the  science  which 


and  both  zenith  and  nadir,  crosses  the  equi- 
noctial at  right  angles,  and  divides  the 
spliere  into  two  hemispheres,  the  eastern 
and  the  western  ;  it  has  its  poles  in  the 
east  and  west  points  of  the  horizon.  It  is 
called  meridian,  because,  when  the  sun 
comes  to  the  south  part  of  this  circle,  it 
is  then  mid-day  ;  and  then  the  sun  has  his 
greatest  altitude  for  thn*.  day. 

METEOR,  in  physiology,  a  moveable  ig- 
neous body,  congregated  in  the  air  by  means 
not  thoroughly  understood,  and  varying 
reatly  in  size  and  raoidity  of  motion. 

METEOROLOGY*  is  the  science  of 
studying  the  phenomena  of  the  atmo- 
sphere, and  that  term  by  which  is  expressed 
all  the  observations  that  tend  to  make  them 
a  system. 

MICROSCOPE,  in  opticks.  By  micro- 
scopes are  understood  instruments,  of  what- 
ever structure  or  contrivances,  that  can 
make  small  objects  appear  larger  than  they 
do  to  the  naked  eye. 

MINUTE,  in  geometry,  the  sixtieth  part 
of  a  degree  of  a  circle.  Minutes  are  denot- 
ed by  one  acute  accent,  thus  (') ;  as  the  se- 
cond, or  sixtieth  part  of  a  minute,  is  by 
two  such  accents,  thus  (") ;  and  the  third 
by  three  ("'). 

MIRRORS,  in  catopticks,  any  polished 
body  impervious  to  the  rays  of  light,  and 
which  reflects  them  equally.  Mirrors  were 
anciently  made  of  metal  ;  but  at  present 
they  are  generally  smooth  plates  of  glass, 
tinned  or  quick-silvered  on  the  back  part, 
and  called  looking-glasses.  The  doctrine 
of  mirrors  depends  wholly  on  that  funda- 
mental law,  that  the  angle  of  reflection  is 
always  equal  to  the  angle  of  incidence. 

MOBILITY  is  that  property  of  matter 


by  which  it  is  capable  of  being  moved  from 
treats  of  the  laws  of  the  equilibrium  andlone  part  of  space  to  another, 
motion  of  solid  bodies  ;  of  the  forces  by'     MOMENTUM,  in  mechanicks,  signifies 


.vhich  bodies,  whether  animate  or  inani- 
mate, may  be  made  to  act  upon  one  ano- 
ther ;  and  of  the  means  by  which  these  may 
be  increased,  so  as  to  overcome  such  as  are 
most  powerful. 


the  same  with  impetus,  or  quantity  of  mo- 
tion in  a  moving  body  ;  which  is  always 
equal  to  the  quantity  of  matter  multiplied 
into  the  velocity  ;  or,  v/hich  is  the  same 
thing,  it  may  be  considered  as  a  rectangle 


MEDIUM,  in  philosophy,  that  space  or[under  the  quantity  of  matter  and  velocity, 
region  through  which  a  body  in  n»otion  MONSOON,  in  physiology,  a  species  of 
passes  to  any  point ;  thus  ether  is  siippos- wind,  in  the  East  Indies,  which  for  six 
ed  to  be  the  medium  through  which  the i  months  blows  constantly  the  same  way, 
heavenly  bodies  move;  air,  the  medium  and  the  contrary  way  the  other  six  months.. 


•wherein  bodies  move  near  the  eartk  ;  wa- 
ter, the  medium  wherein  fishes  live  and 
move;  and  glass  is  also  a  medium  of  light, 
as  it  aflfords  it  a  free  passage. 

MELODY,  in  musick,  the  agreeable  ef- 


MOON,  in  astronomy,  a  satellite,  or  se- 
condary planet,  always  attendant  on  our 
earth. 

MOTION  is  defined  to  be  the  continued 
and  successive  change  of  place.    Nothir 


feet  of  different  sounds,  ranged  and  dispos-  can  be  produced  or  destroyed  without  mo- 


ed  in  succession  ;  so  that  melody  is  the  ef- 
fect of  a  single  voice  or  instrument,  by 
which  it  is  distinguished  from  harmony. 

MERCURY,  in  astronomy,  is  a  small 
star  that  emits  a  veiy  bright  white  light — 
though,  by  reason  of  his  always  keeping 
near  the  sun,  he  is  seldom  to  be  seen  ;  and 
•when   he  does  make  his  appearance,  hi 


tion,  and  every  thing  that  happens  depends 
on  it. 

MUSICK.  Any  succession  of  sounds, 
however  much  they  may  vary  in  regard  to 
duration,  or  however  much  they  may  par- 
take of  various  modes  or  keys,  provided  that 
succession  be  agreeable,  and  excites,  in  a 
■ell  tuned  ear,  certain  agreeable  scnsa- 


motion  towards  the  sun  is  so  swift,  that  heitions,  is  called  musick. 

ean  only  be  discerned  for  a  short  time.  NADIR,  in  astronomy,  that  point  of  the 

MERIDIAN,  in  astronomy,  a  great  cir-  heavens  which  is  diametrically  opposite  to 

«le  passing  tlirough  the  poles  of  the  world,!  the  zenith,  or  poiat  directly  over  our  heada^ 


248 


A   DICTIONARY    OP 


The  zenith  and  nadir  are  the  two  poles  of  jing  from  the  section  of  a  cone,  when  cut  by 
the  horizon.  la  plane  parallel  to  one  of  its  sides. 

NATURAL  PHILOSOPHY,  otherwise!  PARADOX,  in  philosophy,  a  proposition 
called  physicks,  is  that  science  which  con-!seemingly  obscure,  as  being  contrary  to 
siders  the  powers  of  nature,  the  properties  some  received  opinion,  but  yet  true  in  fact. 


of  natural  bodies,  and  their  actions  upon 
one  another. 

NEBULA,  in  astronomy,  luminous  spots 
in  the  heavens,  some  of  which  consist  of 
clusters  of  telescopick  stars,  others  appear 
as  luminous  spots  of  different  forms.  Some 
of  them  form  a  round  compact  system, 
others  are  more  irregular,  of  various  forms, 
and  some  are  long  and  narrow. 

NIGHT,  that  part  of  the  natural  day 
during  which  the  sun  is  underneath   the 


PARALLAX,  in  astronomy,  denotes  i 
change  of  the  apparent  place  of  any  hea- 
venly body,  caused  by  being  seen  from  dif- 
ferent points  of  view;  or  it  is  the  difference 
between  the  true  and  apparent  distance  of 
any  heavenly  body  from  the  zenith. 

PARALLEL  straight  lines,  whose  least 
distances  from  each  other  are  every  where 
equal,  are  said  to  be  parallel. 

PARALLELOGRAM,  in  geometry,  a 
quadrilateral  right  lined  figure,  whose  op- 


horizon  ;  or  that  space  wherein  it  is  dusky,  jposite  sides  are  parallel  and  equal  to  each 

NODES,  in   astronomy,  the   two  points  lother. 
wherein  the  orbit  of  a  planet  intersects  the  j     PARHELIUM,    or    PARHELION,    in 
ccliptick,  whereof  the  node,  where  the  node, physiology,  a  mock  sun,  or  meteor,  in  form 
ascends  northwards,  above  the  plane  of  the!  of  a  very  bright  light,  appearing  on  one 
ecliptick,  is  called  the  ascending  node;  andjside  of  the  sun. 

the  other,  where  the  planet  descends  to  the]     PEGASUS,  in  astronomy,  a  constellation 
south,  is  called  the  descending  node.  |of  the  northern  hemisphere,  in  form  of  a 

OBLATE,  flattened,  or  siiortened,  as  an 'flying  horse, 
oblate  spheroid,  having  its  axis  shorter]  PENDULUM,  in  mechanicks,  denotes 
than  its  middle  diameter,  being  formed  by  any  heavy  body  so  suspended  as  that  it 
the  rotation  of  an  ellipse  about  the  shorter  jmay  vibrate  or  swing  backwards  and  for- 
axis.  The  oblateness  of  the  earth  refers  to  I  wards,  about  some  fixed  point,  by  the  force 
the  diminution  of  the  polar  axis  in  respect  of  gravity.    The  vibrations  of  the  pendu- 


of  the  equatorial, 

OBTUSE,  signifies  hlunt  or  dull,  in  op- 
position to  sharp  or  acute.  Thus  we  say 
an  angle  is  obtuse  if  it  measures  more  than 
ninety  degrees. 


lum  are  called  its  oscillations. 

PENUMBRA,  in  astronomy,  a  partial 
shade  observed  between  the  perfect  shadow 
and  the  full  light,  in  an  eclipse. 

PERCUSSION,  in  mechanicks,  the  im- 


OCCIDENT,  in  geography,  the  westernlpression  a  body  makes  in  falling  or  striking 
quarter  of  the  horizon,  or  tliat  part  of  the|upon  another,  or  the  shock  of  two  bodieg 
horizon   where  the  ecliptick,  or   the  sun  in  motion. 

therein,  descends  into  the  lower  hemi-  PERIHELIUM,in  astronomy,  that  point 
sphere,  in  contradistinction  to  orient.  |of  a  planet's  or  comet's  orbit  wherein  it  is 

OCCULT  ATION,    in    astronomy,    thean  its  least  distance  from  the  sun;  iu  which 
time  a  star  or  planet  is  hidden  from  our  sense  it  stands  in  opposition  to  aphelium. 
sight,  by  the  interposition  of  the  moon  or}     PERIMETER,  in  geometry,  the  boundr 
of  some  other  planet.  lor  lim.its  of  any  figure  or  body.    The  peri- 

OPACITY,  in  philosophy^  a  quality  of  jmeter  of  surfaces  or  figures  are  lines,  those 
bodies  which  renders  them  impervious  to  of  bodies  are  surfaces.  In  circular  figures, 
the  rays  of  light.  [instead  of  perimeter,  we  say  circumference, 

OPTICKS,  the  science  of  vision,  includ-jor  periphery, 
ing  Catoptricks  and  Dioptricks,  and  evenj     PERIOD,  in  astronomy,  the  time  taken 
Perspective  ;  as  also  the  whole  doctrine  of  up  h/  a  star  or  planet  in  making  a  revolu- 
light  and  colours,  and  all  the  phenomena ition  round  the  sun  ;  or  the  duration  of  its 
of  visible  objects.  course  till  it  return  to  the  same  point  of  its* 

ORBIT,  in  astronomy,  the  path  of  a  pla-jorbit. 
net  or  comet,  or  the  curve  that  it  describes]     PERIPHERY,  m  geometry,  the  circum- 
in  its  revolution  round  its  central  body,  ference  of  a  circle,  ellipsis,  or  any  other  re- 
Thus  the  earth's  orbit  is  the  curve  whichjgular  curvilinear  figure, 
it  describes  in  its  annual  course,  and  usu-i     PERPENDICULAR,  in  geometry, aline, 
ally  called  the  ecliptick.  jfalling  directly  on  another  line,  so  as  to 

ORION,  in  astronomy,  a  constellation  of  make  equal  angles  on  each  side;  called 
the  southern    hemisphere,  consisting    of  also  a  normal  line. 

thirty-seven  stars,  according  to  Ptolemy;  PERSPECTIVE,  the  art  of  represent- 
of  sixty-two,  according  to  Sycho ;  aiul  of  ing.  upon  a  plane  surface,  the  appearance 
no  leas  than  eighty,  in  the  Britannick  cata-lof  objects,  however  diversified,  similar  to 
logue.  jthat  tiiey  assume  upon  a  glass-pane,  inter- 

ORRERY,  a  curious  machine  for  repre- 'posed  between  them  and  the  eye  at  a  given 
sent  ing  the  motions  and  appearances  of  the  {distance, 
heavenly  bodies.  PHASES,  in  astronomy,  the  several  ap 

OSCILLATION,  in  mechanicks,  the  vi-jpearances  or  quantities  of  illumination  of 
brat  icn  or  reciprocal  ascent  and  descent  of  jthe  Bloon,  Venus,  Mercury,  and  the  othei 
a  pendnlnm.  planets  ;  or  the  several  mar.ners  whereir 

PARABOLA,  in  geometry, a  figure  aris-lthey  appear  illuminated  by  the  sim. 


PHILOSOPHICAL    TERMS. 


249 


^  PHOENIX,  in  astronomy,  one  of  the 
constellations  of  the  southern  hemisphere, 
unknown  to  the  ancients,  and  invisible  in 
our  northern  parts.  It  is  said  to  consist  of 
thirteen  stars. 

PHYSICKS,  a  term  made  use  of  for  na- 
tural philosophy,  explains  the  doctrines  of 
natural  bodies,  their  phenomena,  causes, 
and  effects,  with  the  various  effections, 
motions,  and  operations. 

PISTON,  in  pump-work,  is  a  short  cylin- 
der of  metal,  or  other  solid  substance,  fitted 
exactly  to  the  cavity  of  the  barrel  or  body 
of  the  pump.  There  are  two  kinds  of  pistons 
used  in  pumps,  the  one  with  a  valve,  and 
the  other  without  a  valve,  called  a  forcer. 

PLANE,  in  geometry,  denotes  a  plain 
surface,  or  one  that  lies  evenly  between  its 
bounding  lines — and  as  a  right  line  is  the 
shortest  extension  from  one  point  to  ano- 
ther, so  a  plain  surface  is  the  shortest  exten 
sion  from  one  line  to  another. 

PLANET,  a  celestial  body  revolving 
round  the  sun,  as  a  centre,  and  continually 
changing  its  position,  with  respect  to  the 
fixed  stars  ;  whence  the  name  planet,  which 
is  a  Greek  word  signifying  wander. 

PLEIADES,  in  astronomy,  an  assem 
blage  of  seven  stars  in  the  neck  of  the  con 
stellation  Taurus,  the  bull ;  although  there 
are  now  only  six  of  them  visible  to  the  na 
ked  eye.  The  largest  is  of  the  third  mag 
nitude,  called  "  Lucido  pleiadum." 

PNEUMATICKS  is  that  branch  of 
natural  philosophy  which  treats  of  the 
weight,  pressure,  and  elasticity  of  the  air 
with  the  effects  arising  from  them. 

POINT,  in  geometry,  as  defined  by  Eu 
did,  is  a  quantity,  which  has  no  parts,  or 
which  is  indivisible.  Points  are  the  ends 
or  extremities  of  lines.  If  a  point  be  sup 
posed  to  be  moved  any  way,  it  will,  by  its 
motion,  describe  a  line. — Point,  in  physicks, 
is  the  least  sensible  object  of  sight,  marked 
with  a  pen,  point  of  a  compass,  or  the  like. 
Of  such  points  all  physical  magnitude 
consists. 

POLAR,  in  general,  something  relating 
to  the  poles  of  the  world,  or  poles  of  arti- 
ficial globes. 

POLARITY,  the  quality  of  a  thing  con 
sidered  as  having  poles ;  but  chiefly  used 
in  speaking  of  the  magnet. 

POLE,  in  astronomy,  one  of  the  extre- 
mities of  the  axis,  on  which  the  sphere  re- 
volves. These  two  points,  each  ninety  de- 
grees from  the  equinoctial  or  equator,  are 
by  way  of  eminence  called  the  poles  of  the 
world  ;  and  the  extremities  of  the  axis  of 
artificial  globes,  corresponding  to  these 
points  in  the  heavens,  are  termed  the  poles 
thereof. 

POLLUX,  in  astronomy,  a  fixed  star  of 
the  second  magnitude  in  the  constellation 
gemini,  or  the  twins.  The  same  name  i- 
also  given  to  the  hindermost  twin,  or  pos 
terior  part  of  the  same  constellation. 

POWER,  in  mechanicks,  denotes  any 
force,  whether  of  a  man,  a  horse,  a  spring, 
the  wind,  or  water,  wnich  Deing  appiiea  id 
a  machine,  tends  to  produce  motion. 


PRECESSION  OF  THE  EaUINOXES 

is  a  very  slow  motion  of  them,  by  which 
they  change  their  place,  going  from  east  to 
west  or  contrary  to  the  order  of  the  signs. 

PROJECTION,  in  mechanicks,  the  art 
of  communicating  motion  to  a  body,  from 
thence  called  projectile. 

PULLEY,  in  mechanicks,  one  of  the 
mechanical  powers,  called  by  seamen  a 
tackle. 

PUMP,  in  hydraulicks,  a  machine  formed 
on  the  model  of  a  syringe,  for  raising  water. 

PYROMETER,  an  instrument  for  mea- 
suring the  expansion  of  bodies  by  heat. 

QUADRANT  denotes  a  mathematical 
instrument,  of  great  service  in  astronomy, 
and  consequently,  in  navigation,  for  taking 
the  altitudes  of  the  sun  and  stars,  as  also 
for  taking  angles  in  surveying. 

QUADRATURE,  in  geometry,  denotes 
the  squaring  or  reducing  a  figure  to  a 
quare. 

QUADRILATERAL,  in  geometry,  a 
figure  whose  perimeter  consists  of  four 
right  lines  making  four  angles  ;  whence  it 
is  also  called  a  quadrilateral  figure.  The 
quadrilateral  figures  are  either  a  parallelo- 
gram, trapezium,  rectangle,  square,  rhom- 
bus, or  rhomboides. 

RADIATION,  the  act  of  a  body  emitting 
or  diffusing  rays  of  light  all  around,  as 
from  a  centre. 

RADIUS,  in  geometry,  the  semi-diame- 
ter of  a  circle,  or  a  right  line  drawn  from 
the  centre  to  the  circumference. 

RAIN.  Whatever  suddenly  disturbs  the 
heat  or  density  of  the  air,  or  the  electricity 
of  the  clouds,  occasions  the  particles  of 
vapour  to  rush  together,  and  form  drops  of 
water  too  heavy  to  continue  suspended  in 
the  atmosphere.  They  then  fall  in  the 
shape  of  rain,  and  increase  in  size  as  they 
fall  by  combining  with  the  floating  vapours 
as  they  pass  through  them. 

RAINBOW  is  a  meteor  in  form  of  a 
party-coloured  arch,  or  semicircle,  exhibit- 
ed only  at  the  time  when  it  rains.  It  is 
always  seen  in  that  point  of  the  heavens 
which  is  opposite  to  the  sun,  and  is  occa- 
sioned by  the  refraction  and  reflection  of 
his  rays  in  the  drops  of  falling  rain. 

RAREFACTION,  in  physicks,  is  the 
making  a  body  to  expand,  or  occupy  more 
room  or  space,  without  the  accession  of 
new  matter. 

RAY,  in  opticks,  a  beam  of  light,  emitted 
from  a  radiant  or  luminous  body. 

REACTION,  in  physiology,  the  resist- 
ance made  by  all  bodies  to  the  action  or 
impulse  of  others,  that  endeavour  to  change 
its  state,  whether  of  motion  or  rest. 

RECEIVER,  in  pneumaticks,  a  glass 
vessel  for  containing  the  thing  on  which 
an  experiment  in  the  air  pump  is  to  be 
made. 

RECTANGLE,  in  geometry,  the  same 
with  a  right  angled  parallelogram. 

REFRACTION,  is  the  deviation  of  a 
moving  body  from  its  direct  course,  occa- 
sluned  by  the  di/TerenL  density  of  the  medi- 
um ill  which  it  moves ;  or,  it  is  a  pha.ngft 


250 


A    DICTIONARY    OF 


of  direction,  occasioned  by  a  body's  falling' at  the  equinox,  where  the  San  intersects 
obliquely  out  of  one  medium  into  anotheri and  rises  above  the  equator,  have  these 
of  a  different  density.  names  and  marks  : 

REPULSION,inphysicks,that  property  A,-  „       cko     t ->«  r\  a      ..4.     ■  *, 

in  bodies,  whereby.  If  Ihey  are  placed  jus'tr^"^''      ^      ^^°'         ^  Sagittarius,    / 
'   ■        •  ^      .      .     .  Taurus,    y      Virgo,     TTJ  Capricornus,Vf 

Gemini,   JJ     Libra,     £v  Aquarius,       -cji 


beyond  the  sphere  of  each  other's  attraction 
of  cohesion,  they  mutually  fly  from  each 
other. 

RESISTANCE,  in  philosophy,  denotes, 
in  general,  any  power  which  acts  in  an  op- 


Cancer,  (^2    Scorpio,  rH  Pisces, 


H 


Of  these  signs,  the  first  six  are  called  north- 


posite  direction  to  another,  so  as  to  destroyern,  lying  on  the  north  side  of  the  equator  ; 
or  diminish  its  effects.  |and  the  last  six  are  called  southern,  being 

RETINA,  the  expansion  of  the  optickj  situated  to  the  south  of  the  equator, 
nerve  on  the  internal  surface  of  the  eye,j  SIPHON,  or  Syphon,  in  hydraulicks,  a 
whereupon  the  images  of  objects  being, bended  pipe,  one  end  of  which  being  put 
painted,  are  impressed,  and  by  that  raeanS|into  a  vessel  of  liquor,  and  the  other  hang- 
conveyed  to  the  common  sensory  in  theling  out  of  the  said  vessel  over  another,  the 
brain,  where  the  mind  views  and  contem-  liquor  will  nwi  out  from  the  first  into  the 


plates  their  ideas, 

ROTATION,  in  geometry,  a  term  chiefly 
applied  to  the  circumvolution  of  any  sur- 
face round  a  fixed  and  immoveable  line, 
which  is  called  the  axis  of  its  rotation,  anJ 
by  such  rotations  it  is  that  solids  are  con- 
ceived to  be  generated 

SAGITTARIUS,  the  archer,  in  astrono- 
my, the  ninth  sign  of  the  zodiack. 

SATELLITES,  in  astronomy,  are  cer 
tain  secondary  planet.-^,  moving  round  the 
other  j)lanets,  as  the  Moon  docs  round  the 
Earth.  They  are  so  called,  because  they 
always  attend  them,  and  make  the  tour 
about  the  sun  with  them. 

SATURN  is  a  very  conspicuous  planet, 
though  not  so  brilliant  as  Jupiter. 

SEGMENT  OF  A  CIRCLE,  in  geometry, 
that  part  of  the  circle  contained  between  a 
chord  and  an  arch  of  the  same  circle. 

SEMICIRCLE,  in  geometry,  half  a  cir- 
cle, or  that  figure  comprehended  between 
the  diameter  of  a  circle  and  half  the  cir- 
cumference. 

SEMIDIAMETER,  half  the  diameter, 
or  a  right  line  drawn  from  the  centre  of  a 
circle,  or  sphere,  to  its  circumference  ;  be 
ing  the  same  with  what  is  otherwise  called 
the  radius. 

SEXTANT,  in  mathematicks,  denotes 
the  sixth  part  of  a  circle,  or  an  arch  com 
prehending  sixty  degrees.  The  \vord  sex 
tant  is  more  particularly  used  for  an  astro- 


last,  after  the  air  has  been  sucked  out  of 
the  external  or  lower  end  of  the  siphon, 
and  that  as  long  as  the  liquor  in  the  upper 
vessel  is  above  the  upper  orifice  of  the  si- 
phon. 

SKY,  the  blue  expanse  of  air  and  atmo- 
sphere. The  azure  colour  of  the  sky  is  at- 
tributed, by  Sir  Isaac  Newton,  to  vapours 
beginning  to  condense  there,  and  which 
have  got  consistence  enough  to  reflect  the 
most  dexible  rays. 

SNOW,  a  v.e!!  known  substance,  formed 
by  the  freezing  of  the  vapours  in  the  at- 
mosphere. It  differs  from  hail  and  hoar- 
frost, in  being  as  it  were  crystallized,  which 
they  are  not. 

SOLID,  in  philosophy,  a  body  whose 
parts  are  so  firmly  connected  together,  as 
not  to  give  way  or  slip  from  each  other  up- 
on the  smallest  impression  ;  in  which  sense 
solid  stands  opposed  to  fluids. 

SOLAR,  something  belonging  to  the  sun ; 
thus  the  solar  system  is  that  system  of  the 
world  wherein  the  heavenly  bodies  are 
made  to  revolve  round  the  sun  as  the  cen- 
tre of  their  motion. 

SOLSTICE,  in  astronomy,  that  time 
when  the  sun  is  in  one  of  the  solstitial 
points  ;  that  is,  when  he  is  at  his  greatest 
distance  from  the  equator,  thus  called,  be- 
cause he  then  appears  to  stand  still,  and 
not  to  change  his  distance  from  the  equa- 
tor for  some  time ;  an  appearance  owing 


nomical  instrument  made  like  a  quadrant,!  to  the  obliquity  of  our  sphere,  and  to  which 
excepting  that  its  limb  only  comprehendskhose  living  under  the  equator  are  stran- 
sixty  degrees.     The  use  and  application  of  [gers. 

the  sextant  is  the  same  with  that  of  the!  SOUND.  The  sense  of  bearing  is  affect- 
quadrant.  |ed  by  the  pulsations  or  vibrations  of  the 

SHADOW,  in  opticks,  a  privation  or  di-| air,  which  are  caused  by  its  own  expan- 
minution  of  light  by  the  interposition  of  anjsion,  or  by  the  vibrations  of  sounding  bo- 
opaque  body;  or  it  is  a  plane,  where  the  dies.  Theae  sensations,  or  vibrations  in 
light  is  either  altogether  obstructed,  or  the  air,  are  called  sounds,  as  are  also  the 
greatly  weakened,  by  the  interposition  of  Isensations  which  they  produce, 
some  opaque  body  between  it  and  the  lumi-j  SPECIFICK,  in  philosophy,  that  which 
nary.  lis  peculiar  to  any  thing,  and  distinguishes 

SIDEREAL  DAY,  is  the  time  in  whiclf  it  from  all  others. 


any  star  appears  to  revolve  from  the  meri- 
dian to  the  meridian  again. 

SIGNS,  in  astronomy.     The  ecliptick  is 


SPECTRUM,  in  opticks.  When  a  ray 
of  light  is  admitted  through  a  small  hole, 
and  received  on  a  white  surface,  it  forms  a 


usually  divided,  by  astronomers,  into  12  luminous  spot.  If  a  dense,  transparent  bo- 
parts  called  signs,  each  of  which  of  coursejdy  1m»  intprpocorJ,  tHn  HgrKt  will  bo  rofractpd, 
contains  30  degrees.  They  are  usually  1  in  proportion  to  the  density  of  the  medium  : 
«all«d  isigns  of  tho  zodiaok  ;  aud  begiuning'but   if  a  triangular  glass   prism   be   inter- 


PHILOSOPHICAL  TERMS.  251 


posed,  the  light  is  not  merely  refracted, 
but  it  is  divided  into  seven  different  rays. 
This  image  is  called  the  spectrum,  and 
from  its  being  produced  by  the  prism,  the 
prismatick  spectrum 


THUNDER,  the  noise  occasioned  by  the 
explosion  of  a  flash  of  lightning  passing 
through  the  air ;  or  it  is  tliat  noise  which 
is  excited  by  a  sudden  explosion  of  electri- 
cal clouds  which  are  therefore  called  thun- 


SPHERE  is  a  soli'l  contained  under  onejder  clouds, 
uniform  round  surface,  such  as  woukl  be|     TORRID  ZONE,  among    geographers, 
formed  by  the  revolution  of  a  circle  aboutidenotes  that  space  of  the  earth's  surface 
the  diameter  thereof,  as  an  axis.  included  between  the  tropicks. 

SPHEROID,  in  geometry,  a  solid,  ap-l  TRADE  WINDS  denote  certain  regular 
preaching  to  the  figure  of  a  sphere.  Iwinds   at  sea,  blowing  either  constantly 

SPOTS,  in  astronomy,  certain  places  of  the  same  way,  or  else  alternately,  a  certain 
the  Sun's  or  Moon's  disk,  observed  to  be  length  of  time  in  one  direction,  and  then 
either  more  bright  or  darker  than  the  rest,|as  long  in  an  opposite  one.  They  are  call- 
and  accordingly  called  facula  and  macula,  ed  trade  winds  from  their  use  in  navigation, 

SPRAY,  the  sprinkling  or  foam  of  the j and  are  very  common  in  the  Indian  seas, 
sea,  which  is  driven  from  the  top  of  a  TRANSIT,  in  astronomy,  signifies  the 
wave  in  stormy  weather.  'passage  of  any  planet  just  by,  or  over,  a 

SQUARE,  in  geometry,  a  quadrilateral,  fixed  star,  or  sun,  and  of  the  moon  in  par- 
figure,  both  equilateral  and  equiangular.      ticular,  covering  or  moring  over  any  planet. 

STAR,  in  astronomy,  a  general  name  for!  TRANSMISSION,  in  opticks,  the  act  of 
all  the  heavenly  bodies  which  are  dispersed! a  transparent  body  passing  the  rays  of 
throughout  the  whole  heavens.  I  light  through  its  substance,  or   suffering 

SUCTION,  the  act  of  sucking  or  draw-|  them  to  pass;  in  which  sense  the  word 
ing  up  a  fluid,  as  air,  water,  milk,  or  the'stands  opposed  to  reflection, 
like,  by  means  of  the  mouth  and  lungs.        I     TRANSPARENCY,  in  physicks,  a  qua- 

SUN,  in  astronomy,  the  most  conspicu-jlity  in  certain  bodies,  whereby  they  give 
ous  of  the  heavenly  bodies,  which  occupies  passage  to  the  rays  of  light,  in  contradis- 
the  centre  of  the  system  which  compre- tinction  to  opacity,  or  that  quality  of  bo- 
hends  the  earth,  the  primary  and  secondary; dies  which  renders  them  impervious  to  the 
planet^,  and  the  comets.  |ravs  of  light. 

SUPERFICIES,  or  surface,  in  geometry,}     TRIANGLE,  in  geometry,  a  figure  of 
a  Magnitude  considered  as  having  two  di-  three  sides  and  three  angles, 
meusions  ;    or    extended    in    length    and|     TROPICKS,  in  astronomy,  and  geogra- 
breadth,  but  without  thickness  or  depth,      phy,  are  two  circles  supposed  to  be  drawn 

SWiMMIxVG,  the  art  or  act  of  sustain-; round  the  earth  on  each  side  of  the  equa- 
ing  ana  moving  the  body  in  water.  Brutes  tor,  and  23  deg,  29'  distant  from  it. 
swim  naturally,  but  men  attain  this  art  by,  TWILIGHT,  that  light,  whether  in  the 
practice  and  industry.  It  consists  princi-' morning  before  sunrise,  or  in  the  evening 
pally  in  striking  the  water  alternately  withiafter  sunset,  which  is  occasioned  by  the 
the  hands  and  feet,  which,  like  oars,  row  a  reflection  of  the  sun's  rays  in  passing 
person  forward.  'through  the  atmosphere. 

SYRINGE,  an  inttrument  serving  to  im-!  VACUUM,  in  philosophy,  denotes  a 
hibe  or  suck  in  a  quantity  of  any  fluid,  and'space  empty  or  devoid  of  all  matter  or  body, 
to  squirt  or  expel  the  same  with  violence. |     VALVE,  in  hydraulicks  and  pneuma- 

SYZYGY,  in  astronomy,  a  term  equally  Iticks,  is  a  kind  of  lid  or  cover,  of  a  tube  or 
used  for  the  conjunction  and  opposition  of  I  vessel,  so  contrived  as  to  open  one  way  ; 
a  planet  with  the  sun.  | but  which  the  more  forcibly  it  is  pressed 

TANGENT,  in  geometry,  is  defined,  inthe  other  way,  the  closer  it  shuts  theaper- 
general,  to  be  a  right  line,  which  touchesjture,  so  that  it  either  admits  the  entrance 
any  arch  of  a  curve,  in  such  a  manner,  asof  a  fluid  into  the  tube  or  vessel,  and  pre- 
to  make  a  right  angle  with  the  diameter  of  j  vents  its  return,  or  admits  its  escape,  and 
the  circle  of  which  that  arch  is  a  part.        prevents  its  re-entrance. 

TANTALUS'  CUP,  in  hydraulicks,  a'  VAPOUR,  in  meteorology,  a  thin,  humid 
siphon,  so  adapted  to  a  cup,  thac  the  short  matter,  which,  being  rarefied  to  a  certain 
leg  being  in  the  cup,  the  long  leg  may  go  degree  by  the  action  of  heat,  ascends  to  a 
down  through  the  bottom  of  it.  jparticular  height  in  the  atmosphere,  where 

TAUilUS,  in  astronomy,  one  of  the  it  is  suspended,  until  it  returns  in  the  form 
twelve  signs  of  the  zodiack,  the  second  in  of  dew,  rain,  snow,  or  hail, 
order,  consisting  of  forty-four  stars,  accord-|  VELOCITY,  swiftness,  or  that  affection 
ing  to  Ptolemy;  of  forty-one,  according  toof  motion,  whereby  a  moving  body  is  dis- 
Tycho  ;  and  of  no  less  than  one  hundred  posed  to  run  over  a  certain  space  in  a  cer- 
and  thirty-five,  according  to  the  Britannick  tain  time, 
catalogue.  |     VENUS,  the  most  beautiful  star  in  the 

TELESCOPE,  an  optical  instrument,  heavens,  known  by  the  names  of  the  morn- 
which  is  used  for  discovering  and  viewinging  and  evening  star,  likewise  keeps  near 
distant  objects,  either  directly  by  glasses,  the  sun,  though  she  receies  from  him  al- 
or  by  retiectior^.  jmost  double  the  dist^.^ice  of  ulercury. 

THERMOMETER,  an  instrument  fori  VESTA,  one  of  the  "^mall  planetary  bo- 
measuring  the  degree  of  heat  gr  cold  in  any,  dies  discovered  lately  lo  revolve  between 
y>odj.  |the  planets  Mars  and  Jupiter, 


^2  A    DICTIONARY    OF    PHILOSOPHICAL    TERMS. 

VIBRATION,  in  mechanicks,  a  regular  jgular  prism,  whose  bases  are  equilateral 
reciprocal  motion  of  the  body,  as,  for  ex-  acute  angled  triangles, 
ample,  a  pendulum,  which,  being  freely  SUS-;     WEEK,  in  chronology,  a  division  of 
pended,  swings  or  vibrates    from   side  to^time  comprising  seven  days, 
aide.  WEIGHT,  in  physicks,  is  a  quality  in 

VIRGK),  in  astronomy,  one  of  the  signs'natural  bodies,  by  which  they  tend  towards 
or  constellations  of  the  zodiack,  and  theUhe  centre  of  the  earth, 
sixth  according  to  order.  I     WHEEL,  one  of  the  six  powers  of  me- 

VISIBLE,  something  that  is  an  object  chanism ;  and,  without  doubt,  contributes 
of  sight  or  vision,  or  something  whereby  more  than  any  of  the  other  live  to  the  ge- 
the  eye  is  affected,  so  as  to  produce  a  sen-iieral  convenience  of  mankind,  by  the  won- 
sation.  'derful  variety  of  purposes,  from  a  mill  to  a 

VISION  is  the  act  of  seeing  or  of  per-j  watch,  wherein  it  is  employed, 
ceiving  external  objects  by  the  organ  ofj     WHIRLWINDS  are  formed  by  opposite 
sight.  j  winds  meeting  and  moving  swiftly  inacir- 

UNDJJLATION,  in  physicks,  a  kind  of  cle,  raising  sand  and  light  bodies  into  the 
tremulous  motion  or  vibration  observable  [air.  In  tlie  deserts  of  Africa  they  some- 
in  a  liquid,  whereby  it  alternately  rises  andjtimes  draw  up  tlie  sand  into  a  moving  pil- 
falls  like  the  waves  of  the  sea.  liar,  which  buries  all  in  its  way.     When 

UNISON,  in  musick,  the  eftoct  of  two'.they  appear  on  the  ocean,  they  draw  up  the 
sounds  which  are  equal  in  degree  of  tune,  water,  and  produce  water-spouts. 
or  in  point  of  gravity  and  acuteness.  |     WIND.     When  the  air  over  anyplace 

VOLCANOES,  mountains  which  emit, is  more  heated  than  that  around,  it  is  rare- 
ignited  matter  and  smoke  through  aper-|fied  or  expanded,  and  rises.  The  surround- 
tures,  communicating  with  cavities  in  thejin^  air  rushes  in  to  supply  its  place,  and 
depths  of  the  earth.  'this  produces  a  current  called  wind. 

VVATER,  a  transparent  fluid,  without]     YEx\R,  the  time  that  the  sun  takes  to  go 
colour,  smell,  or  taste,  in  a  very  small  ue,  through  the  twelve  signs  of  the  zodiack. 
gree  compressible;  and,  when   pure,  not;     ZENITH,   in    astronomy,  the  vertical 
liable  to  spontaneous  change.  j  point ;  or  a  point   in  the  heavens  directly 

WATER  SPOUT,  an  extraordinary  me-  over  our  heads.  The  zenith  is  called  the 
teor,  in  which  a  column  of  water  is  seen  pole  of  the  horizon,  because  it  is  ninety  d«- 
hanging  from  the  clouds,  and  descending  grees  distant  from  every  point  of  that  fir- 
until  it  meets  with  a  column  rising  from  cle. 

the  ocean.  They  unite  and  ofteii^  move  ZODIACK,  in  astronomy, abroad  circle, 
with  rapidity,  until  they  meet  with  some  whose  middle  is  the  ecliptick,  and  its  ex- 
opposing  wind,  or  other  cause,  which  de-'tremes,  two  circles,  parallel  thereto,  at 
stroys  them,  jsuch  a  distance  from  it,  as  to  bound  or 

WAVE,  in  physicks,  a  volume  of  water  comprehend  the  excursions  of  the  sun  and 
elevated  by  the  action  of  the  wind,  upon  its'planets. 

surface,  into  a  state  of  fluctuation,  and  ac-|  ZONE,  in  geography  and  astronomy,  a 
companied  by  a  cavity.  division  of  the  terraqueous  globe,  with  re- 

WEDGE,  one  of  the  mechanical  powers,  jspect  to  the  different  degrees  of  heat  found 
as  they  are  culled.    The  wedge  is  a  trian-jin  the  diflferent  parts  of  it. 


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